I'm just going to throw some of my thoughts into the fray here, as plenty of movesets have been covered already.
First, I hate this thing. It's relegated Jumpluff to being nigh-useless, as well as the other speedy support grass types. That said, other grass types with offensive moves make for good counters against those Whimsicotts that have no offensive attacks but rely on leech seed. But of course it gets hurricane for no logical reason other than to attack other grass types. Also, does sap sipper give immunity to leech seed? If so, Pokes with that ability would also make fair counters.
As has been mentioned, Magic Bounce/Magic Coat will allow you to laugh at Whimsicott as you reflect all its tricks right back at it.
Regarding speed, I don't think it's a good idea to sacrifice speed EVs altogether. Whimsicott isn't just a Prankster, it's the fastest Prankster in the entire game, but only if you invest in its speed. Otherwise it will be outsped by the weather genies and (later) Liepard, who admittedly don't have such a diverse set of status moves but do get taunt and are amply capable of subsequently killing it. Other Whimsicott could be a problem as well. Thus some speed EVs might be a good idea.