I think it's Best in Show, but I have a lot of love for A Mighty Wind as well - Fred Willard steals the show in A Mighty Wind.
On the other hand, I watched Waiting for Guffman 2 weeks ago and loved it. The play itself was a little dry (well, drier than usual), but still hilarious.
Off topic, but I'm of the opinion that Fred Willard is terribly underrated. Anyone remember him on Fernwood 2 Night or DC Follies?
No, but any chance I get to see Fred Willard in a film or tv show, I take it. and to quote Fred in
A Mighty Wind: "WHA HAPPEN?"