(Note: It's a hugely limiting question--if I were to pick the single best film of the past 25 years, it wouldn't be American at all, but this film, presently showing at this film festival (catch it if you can, I urge you).)
From Andy Horbal's film blog No More Marriages:
This is a riff on the very similar poll that the New York Times conducted in regards to books a few months ago. You're on your own defining "American," "fiction," and "last 25 years," but I'm going to insist on that word single. It's this restriction that makes this question so interesting to me.