I like the scene from Season 3.4 "Punked"... Kate shows up and he's all dressed up "How do you know you're in love???" She replies... "All the songs make sense". Her answer is so perfect!
I love in "Vampire Weekend" when he dressed as a Space Cowboy. And Alexis' face when he jumps out of the room... hilarious! "A: there are no cows in space and B: didn't you wear that like 5 years ago?"