I wrote this a couple weeks ago and then had some unexpected things to deal with and didn't get them into the thread. I understand if you don't want to consider them, since they're coming in past the deadline, but tl;dr Marill to S- (or A-) tier is the main takeaway here that hasn't been covered by other posters.
Noms:
Marill is the best and most consistent mon on this team all game long and feels imo like an S-tier. I'm not convinced it's worse than Ralts in any single boss fight, at least not in a significant way, and it always contributes and has a really good early game.
Torchic/Ralts/Shroomish in a similar tier to each other (A/B)
Spoink should be one tier lower than Ralts
Text/logs follow below
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So I did a run of a bunch of strong mons: Torchic, Ralts, Marill, Shroomish, and Spoink. I know a lot of you feel settled about Ralts in S but I at least wanted to see for myself, as well as try to confirm or deny other hunches of mine (that Marill and Spoink should be moved up, for instance).
I tried to aim for Texas's level benchmarks while also giving each Pokémon equal experience (letting Marill run ahead in levels while keeping Ralts a little behind). I also maintained the ~9000 exp. deficit that Spoink had upon capture compared to the rest of the team.
These logs are long and probably redundant but I wrote them anyway and wanted to make it as easy to draw comparisons between these Pokémon, as that could reflect on their tiering.
ROXANNE:
Combusken 16
Ralts 13
Marill 15
Shroomish 16
Combusken sweeps easily at 16 with Double Kick. Cannot really do anything before evolving.
Ralts at 13 can theoretically sweep unassisted with a long Double Team (+ Growl) setup and with luck but it didn't happen in my couple of tries. One Geodude attack + one Nosepass attack kill Ralts unless you Growl them.
Shroomish at 16 and Marill at 15 have straightforward sweeps. I also tested them in a much lower-level team and they can both sweep as low as level 10. I think this should be a point in their favor since Roxanne can be a bottleneck if you don't have the right tools to beat her.
It's worth mentioning that this level benchmark is very high. All the available trainers put together have less than like 3600 exp.—if you're doing a two-mon team like, say, Torchic and Ralts, they're gonna be levels 14 and 11 respectively, too weak to do much against Roxanne. I think this should count as a point in favor of plug-n-play mons like Marill and Shroomish who are able to win with minimal investment, since Roxanne is a serious bottleneck if you don't have the right tools. Marill and Shroomish can save you a lot of grinding.
RIVAL 2:
Optional fight, not gonna go too much into it but at the post-Roxanne levels, Marill with Rollout and Combusken with Double Kick sweep effortlessly. Shroomish can win if it gets lucky against Slugma. I couldn't do a sweep from Ralts unassisted but it should be at least theoretically possible with Double Team.
BRAWLY:
Combusken 19
Ralts 16
Azumarill 18
Shroomish 19
Combusken at 19 usually sweeps with Double Kick spam but Makuhita has a chance of causing trouble with Bulk Up or Reversal.
Ralts at 16 has a chance (maybe 50%?) of sweeping with Confusion spam but Makuhita has a chance of messing you up with Bulk Up or Reversal. Confusion barely misses the 2HKO on Makuhita; Makuhita's Vital Throw just barely 2HKOs.
Azumarill at 18 usually sweeps with Rollout. It's Machop that's most likely to cause trouble here, with the slow setup of Rollout letting Machop get use some annoying attacks (namely Seismic Toss). Rollout misses usually ruin the sweep too.
Shroomish at 19 can uuuusually do the long sweep with all four of its moves (Mega Drain, Tackle, Stun Spore, Leech Seed).
RIVAL 3:
Combusken 20
Ralts 17
Azumarill 20
Shroomish 20
Combusken needs luck/support to sweep. In my attempts it was leveling up after beating Lombre and learning Sand Attack, which gives it a path against Marshtomp (lower accuracy during Bide). Marshtomp usually gets it though.
Azumarill sweeps easily if Rollout hits. If it misses, Azumarill can still usually recover the sweep thanks to its huge bulk (for instance, Lombre's Absorb is a 7HKO; Marshtomp's Mud Shot is also a 7HKO after a Defense Curl).
Ralts is outgunned here. Usually can't make it past anyone even with Double Team without a lot of healing/luck (Lombre's Nature Power and Slugma's Ember are both 3HKOs and Ralts takes a whole bunch of Confusions to kill anything, so it would need to get lucky on confuse/misses. Also Nature Power (Swift) and Yawn bypass evasion anyway).
Shroomish is again capable of the long sweep. The healing from Mega Drain keeps up with Lombre's damage output. Leech Seed + Stun Spore + Mega Drain is usually enough to beat Slugma even through some Yawns. In my attempts Marshtomp usually went for Bide which is very easy to play around.
WATTSON:
Combusken 24
Kirlia 20
Azumarill 23
Breloom 24
The problem with any sweep here is that Wattson's Pokémon can all hit hard. None of my pokes can sweep (though I tried pretty hard to make an Azumarill Rollout work). Breloom is the best, usually able to get through 2 or 3 of the enemy mons. Azumarill can usually beat 2 mons with Rollout, but not more. Kirlia needs to get very lucky to beat anything (a crit, confusion hax, Electrike using Howl too much).
Incidentally, just as a funny aside, Azumarill takes electric attacks better than Kirlia.
MAXIE 1:
Combusken 28
Kirlia 24
Azumarill 28
Breloom 28
Pretty high levels here imo. That said, Combusken, Azumarill (either through Rollout sweep or by slugging it out with Bubblebeam), and Breloom (with Bulk Up) all have paths to a sweep, though Mightyena's Sand Attack can mess things up. Kirlia can't really do much against either Mightyena or Camerupt.
FLANNERY:
Combusken 29
Kirlia 25
Azumarill 29
Breloom 28
Spoink 23
Combusken should solo, but can get haxed by Smog, Torkoal crits, etc. I think the way to go is to kill Numel ASAP and set up on Slugma. +6 attack Double Kick and Return each fail to OHKO the Torkoal (Return just barely).
It is possible but inconsistent for Kirlia to solo with Double Team + Calm Mind setup. Kirlia takes too much damage from too many things and basically has to get lucky with the evasion. Quite often Kirlia just dies to Numel.
Azumarill, on the other hand, sweeps very easily. Don't even need Rollout; Azumarill can just Bubblebeam them (though it's nice to get the Rollout OHKO on Torkoal because even the sturdy Azumarill can't take too many Body Slams (para) or Overheats).
Breloom could theoretically sweep but Flannery chooses Overheat too often, I couldn't make it happen.
The fresh Spoink can't do anything too useful. It can sometimes beat Numel or Slugma. Thanks to Thick Fat, it does survive a Torkoal Overheat, for what it's worth.
By the way, the whole team outspeeds Flannery, so getting outsped isn't a concern (good news for the slow Kirlia and Azumarill).
NORMAN:
Combusken 30
Kirlia 26
Azumarill 30
Breloom 29
Spoink 25
Spoink still isn't doing anything yet. Kirlia can't really do much either—needs a lot of luck just to beat Spinda with DTeam+Calm Mind; wasn't able to get further.
Breloom can sweep with some luck; Spinda's Psybeam and Vigoroth's Slashes are problematic for a set-up, but if Breloom gets 6 Bulk Ups, it can OHKO the Slaking with Mach Punch. It can also come in fresh against Slaking and usually win (Bulk Up when Slaking is active; it'll often try to Counter you. You can be screwed by crits and Yawn but usually Slaking is too gung ho about Counter to do anything to you).
Azumarill can also sweep with some luck. Its main weakness, the low speed, starts to cause problems in this fight. If everything behaves you can get the Rollout sweep, but between Spinda's Teeter Dance, Vigoroth's Slash crits, Linoone's flinching Headbutts, and the accumulation of damage throughout the fight, it's rare for Azumarill to make it all the way. It can usually reach Slaking, though. It can also come in fresh against Slaking and usually win, in a similar vein to Breloom above (Dcurl when Slaking is active, Rollout for the win).
Combusken is not quite fast, sturdy, or strong enough to do a solo unassisted. With healing you can do Overheat against Spinda, set up against Vigoroth, and usually carry off the sweep after like three or four Soda Pops. Overheat OHKOs Spinda and Vigoroth and 2HKOs Linoone (near miss on the OHKO) and Slaking.
RIVAL 4:
Blaziken 35
Gardevoir 31
Azumarill 36
Breloom 34
Grumpig 34
Just mentioning that every party member can sweep easily, even Grumpig.
WINONA:
Blaziken 36
Gardevoir 32
Azumarill 37
Breloom 35
Grumpig 35
Winona is a great boss to set up on thanks to Swablu and its weak attacks (even Breloom can Bulk Up through the 4x Aerial Aces), though Perish Song can be annoying.
Blaziken sweeps with Bulk Up + Rock Tomb/Return/Overheat.
Gardevoir sweeps with Calm Mind + Psychic/Thunderbolt
Azumarill sweeps with Dcurl + Rollout. A Rollout miss against Tropius (with Solarbeam) can be problematic, but against the other enemies, Azumarill is bulky enough to have chances to restart the Rollout (Altaria's Dragonbreath is a 6HKO, for instance. Even the Tropius Solarbeam fails to 2HKO more often than not).
Breloom sweeps with Bulk Up + Return/Mach Punch.
Grumpig's sweep is slower and less consistent, but you should at least reach Altaria every time with Cray + Psychic/Psybeam/Shock Wave and have an OK chance of beating Altaria depending on Confuse Ray hax. Grumpig is pretty bulky and Winona is weak. Altaria's Earthquake even after two Dragon Dances is kinda pathetic.
TATE & LIZA:
Blaziken 41
Gardevoir 37
Azumarill 43
Breloom 40
Grumpig 41
Really hard to tell who's best here. I tried a couple different poke orders. Generally Breloom suffers the most due to type disadvantage (Blaziken is also weak but Overheat is very useful) but each team member can contribute something good. Gardevoir Trace on Claydol's Levitate would have been nice but I didn't think about it for this test.
I was able to get one sweep off with Grumpig and Azumarill but it was extremely lucky (enemies hit themselves in confusion almost every turn). Grumpig's Confuse Ray is probably the bigger enabler in this matchup but is, of course, inconsistent.
JUAN:
Blaziken 43
Gardevoir 39
Azumarill 45
Breloom 41
Grumpig 43
Blaziken can set up against Luvdisc and get to Kingdra, but Kingdra's faster and hits too hard for Blaziken to complete the sweep.
Gardevoir and Grumpig have straightforward Calm Mind sweeps. Without Calm Mind, Grumpig can't slug its way to the end, though maybe there are some possibilities I haven't thought of (Rest, Double Team, various berries/hold items to help). Can often get to Kingdra though.
Azumarill has a few ways to execute a Rollout sweep, should be pretty simple as long as it hits. Without Rollout, a Return sweep should be possible though Kingdra's walling and evasion make that up to luck.
Breloom can't do the sweep because it gets outsped by Kingdra and killed by its Ice Beam (+6 Mach Punch doesn't OHKO). I was able to cheese a victory by being at low enough HP going into Kingdra to trick it into using Water Pulse instead, but I don't think that's a strat lol
WALLY:
Blaziken 43
Gardevoir 39
Azumarill 45
Breloom 41
Grumpig 43
Blaziken can't deal with Altaria (Return fails to 2HKO, Altaria 3HKOs you and heals and might set up, too). You can set up on Delcatty with some preparation (Chesto berry to deal with Sing). After that it's simple; +2 Return/Double Kick OHKOs Gardevoir, Roselia, and Magneton.
Like Blaziken, Breloom can't set up on Altaria (4x Aerial Ace almost OHKOs) but can set up on Delcatty afterwards. It also helps that Gardevoir always goes for Future Sight first instead of Psychic, giving the slower Breloom a turn).
This is a good demonstration of Grumpig's strengths compared to Gardevoir's (though if I weren't underleveled Gardevoir would probably do just fine, too). Both OHKO Altaria with a +3 Calm Mind and have some chances of taking the sweep all the way. But Grumpig takes more Aerial Aces, giving it the health to set up a little more against Delcatty and make the eventual enemy Gardevoir kill much more consistent. Grumpig also can use Confuse Ray to make everything easier (notably, Altaria dies to a +2 Psychic + confusion damage, a really useful gained tempo versus the +3 Psychic). At the levels I fought Wally at, this is the difference between a very consistent sweep and one that you need to be very lucky to pull off.
Azumarill usually does it with a couple of Rollouts. Rollout needs to OHKO Magneton (i.e. not miss, be at a high enough power) or else its Thunderbolt stops your sweep. Wally sends in Delcatty at a convenient time to let you restart your Rollout and carry it through to the OHKO on Gardevoir. Without Rollout, I don't think there's a way for Azumarill to either avoir or survive both Magneton's Thunderbolt and Gardevoir's Psychic, so you can't complete the sweep with just Return + Surf. Maybe if I were at a higher level this would work better.
Actually I went ahead and handed out some Rare Candies: Gardevoir at 43 sweeps pretty easily and Azumarill at 49 can sweep with just Return/Surf. And both benchmarks are several levels below Texas's recommendation for the Elite Four (if you bear exp. rate in mind) so I think that's probably a more accurate test, but I'm reaching the end of the run and just trying to finish it lol
SYDNEY:
Blaziken 51
Gardevoir 46
Azumarill 54
Breloom 48
Grumpig 52
Breloom and Blaziken have straightforward sweeps as long as they don't get too haxxed by Mightyena's Sand Attack.
Azumarill almost always gets too haxxed by Sand Attack to pull off a Rollout sweep. It's possible but can't be counted on. If you kill Mightyena with something else, Azumarill can come in on Shiftry and usually get a sweep started, though its own evasion move can make things annoying.
Grumpig and Gardevoir.... it sucks that Cacturne and Shiftry resist the coverage move as well as Psychic lol. Grumpig can usually do a little more with its better bulk and Confuse Ray (with luck, Grumpig can set up on Shiftry, but it almost always gets frustrated somewhere) but they're both bad here.
PHOEBE:
Nobody's great here. Grumpig (Calm Mind x3 + Psychic) and Blaziken (Bulk Up x3 + Earthquake) can sweep with luck. Theoretically Gardevoir should be able to do the same as Grumpig but I couldn't make it happen (Grumpig's Confuse Ray and extra bulk strike again—the extra turn it can get either through confusion or through surviving an extra Shadow Punch can make the difference). Any sweep, of course, can be stopped by Curse.
I couldn't make any progress against the first Dusclops with Azumarill, but if you switch Azumarill (with Sitrus Berry) into the Banette, it'll survive three Thunderbolts (just barely), enough time to do Defense Curl + Rollout and set up the rest of the sweep.
My Breloom can't hit ghosts so ¯\_(ツ)_/¯
GLACIA:
Blaziken has the easiest sweep: +2 Double Kick, and the Sealeo you set up on can't do too much to you. Cheri Berry's good for preventing BSlam para from getting you, which is the only way you can lose here.
Azumarill with Sitrus Berry + DCurl + Rollout pretty consistently got me the sweep here. Azumarill takes a lot of chip damage so the extra HP from Sitrus Berry was often critical.
Gardevoir at +2 can OHKO everything but the Glalies are ranges; ideally you want +3. This should work but you can get haxxed by BSlam para, which will kill your sweep. Grumpig works similarly. At +2, the Glalies are either ranges or Grumpig falls just short of being able to OHKO. At +3, Grumpig OHKOs most of the team but Walrein is another range. Grumpig can survive Walrein's attacks decently well but, again, a BSlam para will kill the sweep. Cheri Berry's a good idea for both of them.
Breloom is another poke frustratingly close to a clean sweep. It would sweep with +1 Sky Uppercut, but the level 52 Glalie outspeeds it. So you need +2 Mach Punch, but that's a range. I didn't manage to find a way to get to +3 consistently, so an almost sweep, but not quite enough.
DRAKE:
The low speed of the team makes this battle really hard. Nobody's really good. Azumarill has a chance of taking down 3/4 pokes with Rollout but that's really inconsistent. I didn't try Ice Beam Azumarill—maybe that'd be alright. Grumpig and Gardevoir are also too outsped to sweep, though they can make it through 2 pokes (3, if you're Grumpig and carry a Sitrus Berry for the health). Breloom's +6 Mach Punch fails to OHKO Flygon so that's too bad for that sweep.
WALLACE:
Only Grumpig is good here imo.
Breloom and Blaziken get killed by everything and are pretty much useless.
Azumarill can kill as many as 2-3 pokes with Rollout, but needs to be lucky to do so and can die as early as Wailord.
Gardevoir can usually get past Wailord but gets outsped by Tentacruel and can't usually continue the sweep.
Grumpig can sweep, but needs to be lucky (namely, a bunch of confusion hax against Wailord). Even when not lucky, Grumpig can often get the 2 or 3 Calm Minds needed to at least get as far as Ludicolo or Milotic.
MY NOMS/RATINGS
I am as usual hesitant to really get behind a tier judgment, but I hope these logs are fairly comprehensive and that they can defend my ratings here.
Marill is the best and most consistent mon all game long among these mons and feels imo like an S-tier.
Torchic/Ralts/Shroomish in a similar tier
Spoink should be one tier lower than Ralts