Toxicroak B- -> B
Let me start out by saying I love Toxicroak, but I won't be using any bias.
Toxicroak is a pretty odd choice in OU, slower speed tier, only decent attack, quad weakness to Psychic, but it does have it's good points. For one it's probably one of the best offensive checks to Belly Drum Azumarill, or Azumarill in general because of it's ability Dry Skin. It also is one of the few physical Pokémon that don't care about Scald. Dry Skin is such a useful ability against Rain teams and against bulky waters. Of course some bulky waters beat it like Slowbro because of it's psychic typing and it's bulk but most bulky waters (notably it walls crocune and crobro) won't enjoy it on your team. Although having a weakness to three common offensive types, - flying, fire and ground - with a quad weakness to a less common one - psychic- is disheartening, but having an immunity - water - and six resistances - bug, poison, fighting, grass, dark, and rock - is also giving it a somewhat decent time for switch ins, it doesn't have the best bulk - 83/65/65 - but given a free switch via Volt/Turn or Dry Pass can be beneficial as this is the best physical poison type you can find - don't quote me on that pls -. It has a boosting option in Swords Dance and a powerful STAB in Gunk Shot, also Cross Chop if you roll like that. It has access to Bullet Punch - which is useless for fairies because Poison STAB, only decent on Diancie weak teams -, and Sucker Punch - which is a required move on croak imo -, Drain Punch is decent too. Although that's all well and good, it's... somewhat mediocre offensive stats of 106/86/85 make it somewhat weak before a Swords Dance. It is also forced out by Altaria after a Dragon Dance (but seriously, that's no surprise) and a few fairies have ways to get around it (flamethrower clef, azumarill just play roughs, fire blast and earthquake on altaria) but given a 1v1 situation it wins almost every time.
I also think that Toxicroak is better than some 'mons like Bronzong. Tangrowth, Tyrantrum, and Mandibuzz for teams because it blanket checks so much and can turn an Azu sweep into your own sweep. I also believe it fits in with Hawlucha, Scolipede, Slowking, and Tentacruel, Hawlucha and Scolipede has the same general effectiveness as Toxicorak after an SD (bar speed) and Slowking and Tentacruel are good for checkign some high tier threats like MMetagross Clefable Keldeo and Azumarill, which Toxicroak does all that bar MMetagross.
http://replay.pokemonshowdown.com/ou-237046309 Here's a replay I have, I 6-0ed him but I know he's a good player, he's a friend of mine that has beaten me a lot. Also ignore the chat, we had some idle conversation and it might be a little offensive to some people heh.
Edit: Please don't menton obvious shit like 'gets forced out by talonflame' because everybody knows that.
AraEdit: Keldeo was moved to A+.
Me Edit: O. Didn't know that. Need to start keeping track of this stuff.