Right, so. I play too much Pokemon for my own good. *cracks knuckles* let's do it (gonna add a "T" to every theory vote).
Absol - can set up and sweep on Shauntal and Caitlin (as long as you're willing to use full restores), and that's where his usefulness ends. Still, not Bottom, but rather Low, because he's usable as soon as you get him (albeit for random trainers mostly).
Audino - yeah, no, I'm not that hardcore (despite having to use this crap in my nuzlocke geez). Theorymoning Bottom. (T)
Banette - good moveset (with TMs at least), but has very weak defenses and speed. It managed to kill Caitlin's Musharna somehow. That's the best I got out of him. Bottom.
Basculin - this thing has great coverage, with above average attack and speed to back it up. Kind of usable special too. The problem is that if it doesn't manage to get OHKO, it will get destroyed, as it has pretty much non-existant defenses. It can be useful however, as it has enough power to OHKO frail pokes due to good coverage (I was running double edge, superpower, aqua tail and crunch). Low.
Bouffalant - not only you get him extremely late, but it's very possible that you won't even have Swords Dance when you do E4. Bottom. (T)
Buizel - lighting fast, high attack, has very nice coverage with crunch/aqua tail/ice punch. It would be strong High, but sadly, it has issues. Very bad bulk, coupled with lack of proper stat boosting move until post game, means it needs to score OHKO or face serious retaliation. For that reason you're pretty much forced to hunt for Buizel without -Atk Nature. If you can do it however, he's great vs Skyla, Drayden, Shauntal, Caitlin (50/50, because you know, Reflect) and Iris. I'd go for Mid.
Buneary - it's not completely useless as it has nice speed and Jump Kick. He can run all elemental punches if you're willing to spend shards on those. What that basically means most of the time is that he comes in, kills something in two or three hits, then needs to be switched out as he's almost dead. Bottom.
Castform - yeah, no, I'm not that crazy.
Cleafairy - great moveset. Of course, you're better off just using Metronome and hoping for Aeroblast or something, lol. Bottom.
Combee - can be pretty good with Quick Claw and Defend Order set-up. Of course that requires setting up on nearly every stronger trainer, praying for luck and not using Lucky Egg. Bottom.
Cottonee - high, because you can trade it for a Petlil ;D Bottom otherwise. (T)
Cubchoo - it's possible to have Beartic before Skylla without too much grinding, and then fun begins. He's also very capable against Drayden's Haxorus, but his other pokes are a gamble (Druddigon carries Revenge, while Flygon has Rock Slide). He can more or less clear the gym for you though. And.. that's pretty much it. Also Cubchoo needs exp share before evolving as he's weak and has crap moves. Low.
Deerling - pretty good actually. Kind of like Buizel (fast, strong, good moveset) but more bulky. Clay and Marlon are his bitches. Very useful against Corless too, not to mention about half of normal Team Plasma trainers. Can take out Grimsley also, however it's not guaranteed. He can also take Caitlin's Musharna and Gothilette rather easily, but Reflect can prove problematic. Not high, because after Clay he will not shine too much until Seigaha and Plasma Frigate. Mid.
To be continued :P