"Zombieland"

Bill Murray's cameo has to be up there with best-on-screen-comedy-cameos-that-centralize-laughter-around-an-entire-movie (also see Will Ferrell in Wedding Crashers). It was like Zombieland suddenly stopped being Zombieland and instead turned into 5 minute comedy relief (not that the movie needed comedy-relieving; pretty entertaining as it was). He doesn't even say that much, but when Bill Murray is dressed like a zombie, it is just funny.