My favorites (in terms of stuff that's just straight comedy):
Shaun of the Dead-This movie is hysterical. Everybody's comedic timing is perfect, and the jokes are all very funny. The characters are very fun to watch, especially Pegg's and Frost's. Plus, it manages to be both a good zombie movie and a good romantic comedy at the same time.
Superbad-This movie makes me laugh so hard. I also think that it's got a lot of heart; it's my personal favorite high school comedy of the decade.
40 Year Old Virgin-I don't count Funny People as a comedy, so I'm choosing this from Apatow's work. Carrell gives a performance that is both funny and likable, and there are so many good lines.
Zoolander-This movie is absurd and hilarious. I honestly can't believe that it exists, really. It's strange as hell.
Step Brothers-Hilarious. No other word for it. It's batshit crazy and insanely quotable.
Hot Fuzz-Like Shaun of the Dead, this manages to be a good cop movie while simultaneously being a good buddy comedy. It's as good as Shaun of the Dead.
Pineapple Express-Best stoner comedy of the decade. Franco is damn funny.