Catching a Huge Power Marill is really easy this time around. The opportunity cost is almost negligible (and hard to really draw).
Okay time to edit the post because there were a couple things Celever wanted to discuss and I would be more than happy to place my input.
Tentacool B --> A
Zubat B --> A
Oddish (Vileplume) C --> B
Swablu C --> B
Feebas C --> D
Electrike A --> S (THIS needs discussion right now, and I will post thoughts on it tomorrow)
Rhyhorn C --> B
Horsea (Trade) B --> C
Magikarp A --> S
Aron B --> A
Slakoth (Slaking) E --> D
Slakoth (Vigoroth) Untested --> C
Castform E --> D
Makuhita (Trade) A --> S
Mawile C --> B/A
Magnemite S --> A (IAR is using it in a run and will keep us updated as he plays through the game. I think S right now)
So I want to point out that a lot of these Pokemon appear later in the game (some exceptions of course). I'm going to break them down little-by-little and post my thoughts.
Zubat from B to A - I am currently using Zubat. It's actually very good when you use it. But I just don't see it surviving later on the game. Though STAB Acrobatics sounds really good for Zubat it isn't available until Level 35 - that's about when you face Winona. Sludge Bomb and Shadow Ball, though helpful coverage moves, kind of sucks when your Special Attack is about 70. Well I guess at least Cross Poison is available for Crobat for what it is worth which runs off of base 90 Attack. It's one of those Pokemon that would prefer to be DexNav'd too. Brave Bird is definitely icing on the cake, but Zen Headbutt bypasses its issue of having no Super Effective attacks against Poison-types, Giga Drain is at least nice to bypass Rock-types, and Nasty Plot is kind of cool since there are a lot of decent special attacks that Zubat family learns (Sludge Bomb, Shadow Ball, Air Cutter, Air Slash is kind of late). I feel it would be more sold being A Tier if it had access to Acrobatics a little earlier or something. I'll continue using it and reporting in. But honestly aside from Brawly it doesn't have huge advantages in important fights (in fact some like Wattson and Tate / Liza it flat out loses).
Swablu - I feel the
only way that Swablu can even dream of becoming B Tier is if you catch Swablu while using the Eon Flute, using a Rare Candy to evolve into Altaria, obtaining Altarianite from Lilycove, and teaching it Dragon Dance through Move Relearner. Maybe Take Down too because Secret Power / Facade are your best Normal-type STABs. Now I know you're asking why under these conditions so I will gladly explain.
Swablu is an atrocious Pokemon. I'm not even trying to beat around the bush by saying this. 40 Attack / Special Attack until Level 35 is downright humiliating. Level 19 Swablu are your best bet. TM-wise you can teach it Aerial Ace and Secret Power to work with. Secret Power helps with status effects - something Swablu desperately needs just to stand out.
Allow me to state it like this -
Celever wanted me to use Swablu and, to be honest, I really don't want to because looking at it on paper alone makes me cringe. Celever - I can test Swablu if you really want me to; however, I'm probably not going to say many good things about it and state that C is the best it is going for. B is only possible, IMO, through what I mentioned above. This is because at least Mega Altaria does modestly against Drake and Sydney. Before you say "lolMegaAltaria will shit on them" - most of Sydney and Drake's Pokemon will likely be faster than said Mega Altaria unless it is trained (plausible) and even then base 70 Speed is mediocre. It ties I believe with Absol and Altaria and only beats Cacturne in Speed. I feel Mega Gyarados with Crunch, Waterfall, Ice Fang, and Waterfall would do more damage to these guys anyway. Or Taunt / Earthquake I guess. You get the point.
Electrike I think the Electrike available around the Lati@s event are more favorable for the player than the ones you get right away. You can catch Electrike at Level 24 and it evolves at Level 26. Coincidentally you will also have access to Mega Manectric at this point as well. Until you have access to Discharge you can use Charge Beam. With Mega Manectric and against a Pokemon with the same defenses Charge Beam will out damage Thunder Fang. To boot you would also gain a Special Attack boost. Another thing you could do is Volt Switch around - this does have some minor hiccups but, honestly, Manectric can realistically always lead a battle and Volt Switch elsewhere. Some Pokemon like Winona's Skarmory have Sturdy anyway so you're not OHKOing it. Speaking of Winona - Manectric trashes this gym like it's nobody's business. The only "moderate" threat is Swellow's Double Team IMO. You also have Overheat to play with as Manectric too - though dangerous due to the SpA drop it allows Manectric to plow through a Grass-type once and then probably Volt Switch to something else (or just switch - either is fine).
I think currently it stands
A. Sad as it sounds I would put Absol in S before this thing after some long thinking.
Vigoroth / Slaking debacle - I'm not really sold on Vigoroth over Slaking. Vigoroth has this issue of having 90 Base Attack for practically forever and so-so moves to boost its offense (Hone Claws, Bulk Up). It's quick but its durability is also so-so (80/80/55). Eviolite helps I guess but eh. I think in the end I still would rather use Slaking - though switching sucks Slaking is almost guaranteed to nuke one Pokemon whereas Vigoroth is probably going to need setup to get through and still not end as expected through its "sweep". People who actually used one or the other are free to chime in here.
That's it for now. I'll add more later.