Stats
Whimsicott, eh? Curious. On the positive side, 116 base Speed outruns a good chunk of the game, and... then there's the rest of the stats. Defensive spread of 60/85/75 is mediocre at the best of times, and a SpAtk of 77 isn't exactly awesome. Atk? People who run physical Whimsicott probably also run physical Blissey.
Movepool
Status-wise, the pool consists of Tailwind, Cotton Spore, Leech Seed, Substitute, Taunt, Fake Tears, Switcheroo, Tickle, Cotton Guard, Helping Hand, Stun Spore and Toxic, with some other TMs in there. Combine that with Prankster, and you can really mess with people's day, by Switcherooing rubbish items onto their 'mon, crippling them with Poison or Paralysis, Taunt and Leech Seed, maxing stats with Tailwind and Cotton Guard, or dropping stats with Fake Tears, Tickle etc., all behind a priority Sub. Special attack-wise, you get Hurricane, Moonblast, Dazzling Gleam, Energy Ball, Solar Beam, Giga Drain, Psychic and Shadow Ball. Handy, but not overly powerful due to the SpAtk. Physically, it gets Seed Bomb, U-turn and Knock Off. Poor, but handy, especially U-turn.
Abilities
Whimsi gets Prankster, Infiltrator and Chlorophyll. Chlorophyll is extremely risky due to the Sun weather often being caused by MegaZard Y. Infiltrator helps screw over Substitute users, like Baton Pass teams. Prankster is the best by far though, as the majority of its decently usable moveset is Status. In short, a Prankster Whimsicott is a usable Whimsicott.
Sets
So Fluffy
Whimsicott@
Bold
252 HP, 128 Def, 128 SpDef
-Substitute
-Leech Seed
-Taunt
-Toxic/Cotton Guard/Tailwind/Moonblast
Basic Subseed set, with Taunt to cover other Taunters. Final slot depends:
Pure evil=Toxic.
Useful defense=Cotton Guard.
Hyperspeed=Tailwind.
An actual damaging move=Moonblast.
Simple, so long as you don't meet Talonflame or MegaZard Y.
Doubles/Triples
Whimsi gets by fairly OK in this style, as it resists Surf and EQ, but has to be wary of Sludge Wave. It just runs the normal sets.
Counters
Poison slaughters the fluffball faster than anything else. Saying that, Scizor walls everything is can use, and OHKOs with Bullet Punch. After that, Talonflame tears it to shreds, and Ice Shard can decimate it.
Note: Whimsicott getting Fairy was a mixed blessing. I mean, losing the weakness to U-turn was good, and the resistances to 3 of the most common attacking types is good. On the other hand, weakness to Aegislash's Iron Head is a problem, and the quad weakness to Poison doesn't help much, especially with the number of NU Poison types and the availability of Sludge Bomb.