Just a small reminder that for these lists we should be considering supportive/defensive viability too because Doubles is played very differently than Singles. typically in the mainline games, hyper offense is the way to go due to the singles format and there are much less stally tactics seen in the maingame. The AI in the Gamecube games are also different than the main series in that they more often then not employ actual strategy. You could even look at certain aspects of Smogon's own Singles and Doubles meta where Deoxys-A for example in singles will never leave Ubers, while in doubles it hangs around UU. Hyper Offense isn't everything in the Gamecube games as the AI do love to gang up on your frailer pokemon in general. Also speaking of doubles, babying Pokemon is also much less of an issue than in mainline games due to the 2v2 aspect so unless the Pokemon is terrible *cough*Ledian*cough*, babying is not that bad of an offset due to the other Pokemon in the spot being an offensive powerhouse. Yes, Espeon is arguably the best Pokemon to used in the game, but even Espeon with its high special attack does fail to secure OHKO's unless its overleveled, and can be frail on the physical side so it's no stranger to being double targeted, so unlike in singles ingame format, supportive and defensive Pokemon do have great use in the ingame of Colosseum.
Anyways I'd like to nominate a few Pokemon.
Quagsire: I think Quag should go in A tier due to its great level up movepool, which makes it so you don't have to use many one off or hard to obtain TMs on it, early encounter with a small shadow gauge a decent enough special attack stat to abuse SE hits, good natural bulk, and a good showing in all major battles, except for Miror B.
Plusle: Some may thing Plusle is outright garbage due to it being obtained at a low level of 13, and that Minum is absent from the game so it technically doesn't have an ability, but don't forget, it's the only Pokemon in the entire game prior to Postgame that receives bonus EXP due to it having a different OT when obtained. So it doesn't take long for it to catch up to your team, as well as it will most likely outlevel your team later on too. It also has an absolutely fantastic level up movepool for doubles with some great moves such as Helping Hand, Fake Tears, Encore, T-Wave, and Quick Attack to snag a few KOs at low percentages, though its TM movepool leaves much to be desired (though it does gain Light Screen), especially since many of Colo's TMs are either Postgame or simply missing in the game in general. I personally had quite a fun time using Plusle when I finally gave it the time of day, and it surprisingly held up well and even bodied Nascour hard in one of my playthroughs. I'd nominate Plusle for B tier due to it taking babying, but it does make up for it when you take the time aside for the Pokemon for its ability to secure crucial KOs with either Helping Hand, Fake Tears or Quick Attack as well as it having Encore that is useful against the more status oriented major battles, and its fast level up rate with it being a "traded" Pokemon.
Slugma: I'd nominate Slugma for E tier. If you saw my analysis for Sudowoodo earlier in the thread, it shares nearly all the same flaws with its rock brethern with some of the flaws being multipled such as being 4x weak to both water and ground, which there are plenty of in this game. As with Sudowoodo it has bad matchups against all of the major battles. It does have some usefulness due to it naturally having both yawn and flamethrower but it also learns its only two level up moves far too late when it needs to and its typically too slow and frail to take advantage of the two good moves it actually has. The only reason it's not F tier imo is because its an early encounter and even with its big flaws, its still infinitely more useful than anything in F tier.
Swablu: I love Swablu and Altaria, one of my favorite Pokemon ever. I was pretty ecstatic when it got a mega in ORAS. Anyways, my love for Swablu aside, I would like to nominate it for B tier. You catch it pretty early, it doesn't take long to purify and only evolves two levels after you get it. When it evolves, it also gains the elusive (for Colosseum standards) Dragon/Flying typing, which does come in handy against the weather teams you will probably face against as well as being immune to the EQ spam you will encounter many times throughout the game. But the main reason why I'm nominating it for B is that it's pretty good at any role you give it. You could make it a pure defensive pokemon/status absorber with its bulk and a great ability in Natural Cure, you could make it a good support mon with moves like sing, dragonbreath, perish song and being the second fastest pokemon you can obtain that can learn safeguard. You could even sweep with Dragon Dance as it has enough bulk to use 2-3. Dragon Dance also makes Swablu/Altaria one of the literal handful of Pokemon that you can obtain that can actually boost one of their offensive stats, at least before the Post Game. Sadly, just because its good at everything, Altaria is not great or fantastic at any of the things it can do. It's not going to be sweeping teams like Espeon with little to no support, or absorbing all the hits to heal up your backrow with items like Umbreon, but its a nice and well rounded fillermon.
