The Cheesing one from 2008 "Major Boobage" was hilarious.
I liked the imaginationland 3 part they did last season too.
Other honorable mentions:
Manbearpig
Scott Tenorman must die
The one with the headlice; Lice Capades (ah kelly, and my unborn BAY BAY!)
The Ginger episode: hilarious
The "Asspen" episode, if you've seen a good amount of movies from the 80's, this episode is even funnier.
The "you got served episode", with butters erratic dancing setting off a chain of events killing the audience, that song he dances to had me crapping my pants;
I've got something in my front pocket for you.
Why don't you reach down in my pocket and see what it is?
Then grab onto it, it's just for you.
Give a little squeeze and say: "How do you do?"
I'm sure there are more funny ones, that show is great. I'm still waiting for a real good one this year, the first two were good, not great.