This is going to sound really, really fucking weird, but stay with me.
Qwilfish for B rank.
In my opinion, Qwilfish is one of the most underrated physical walls in the whole entire game. Max Defense Qwilfish can switch into a V-Create and take like, 20-30%. It can also switch into a Heracross' CC and take like, 17%. In the physically offensive, Fighting-move heavy UU metagame that we currently face, Qwilfish is one of the most useful pokemon I've ever used. Spikes and Toxic Spikes combined with intimidate let it switch in on any physical threat and get up free hazards, and explosion prevents it from being complete set-up fodder. A scenario that plays out in about 15% of my UU games is as follows: Qwilfish switches into Scrafty, intimidates, as Scrafty attacks, does fuck-all damage. Opponent goes "amg free setup1!" and sets up four dragon dances as I get all my hazards up. They crunch at +3/+4, do like 55%, and I explode, which is usually a OHKO. (especially because I run it on a hail team, so residual helps me out)
Qwilfish's one drawback is its pitiful SpD, but honestly, it fills a very important niche role as a sweep stopper and spiker, and can very easily form part of a solid defensive core with something like, say, SpD Lefties Zapdos. Access to Explosion/Destiny Bond and a very respectable 85 speed lets it score surprise kills. It also gets Taunt, Haze, and Thunder Wave.
Qwilfish's typing also ensures that the field will be free of toxic spikes. Water/Poison can also be a very helpful typing in its own merits - if played correctly, Qwilfish can predict a switch into a ground type (Read: Nidos) and fire off a surprisingly powerful base 95 stab Waterfall.
I could go on for a couple more paragraphs about how underrated this ugly spiky poison fish is, but I think I'll just add some calcs to let it speak for itself.
252Atk -1 Heracross (Neutral) Close Combat vs 252HP/252Def Qwilfish (+Def): 16% - 19% (54 - 65 HP).
252Atk -1 Victini (+Atk) V-create vs 252HP/252Def Qwilfish (+Def): 23% - 27% (77 - 91 HP).
252Atk -1 Reckless Mienshao (+Atk) Hi Jump Kick vs 252HP/252Def Qwilfish (+Def): 23% - 27% (78 - 93 HP).
0Atk -1 Heavy Metal Bronzong (Neutral) Earthquake vs 252HP/252Def Qwilfish (+Def): 22% - 26% (76 - 90 HP). (just put this one in here because people like to think they can kill me with bronzong as I set up on them :)
252Atk +3 Scrafty (+Atk) Crunch vs 252HP/252Def Qwilfish (+Def): 72% - 85% (241 - 285 HP).