TV's Frink said:

Rushmore is fantastic. His best film IMO.

It's hard for me to really judge as I've only seen two of his, but from what I've heard most people say the same. Personally, I think I prefer Moonrise Kingdom. I don't think either is necessarily better than the other, but MK is definitely more stylized (which I liked) and has, in my opinion, a more interesting storyline. The characterization in Rushmore is better, so it's a close call. I think MK is my favorite of his (so far).