[Porch Dweller]

I'd probably have to say "Miller's Crossing". IMNSHO it's the Coen Bros best film, has flawless performances, the best Tommy Gun scene ever (bonus for Albert Finney restarting his cigar at the end of the scene), some genuinely laugh out loud moments, a complex but accessible plot, some shocking twists and turns, and features some gorgeous cinematography.
The first time I saw it I found it a bit cold and dour. It wasn't until subsequent viewings that I appreciated it fully.
And I know some consider it heresy to say this, but it's as good if not better a gangster film than "The Godfather".

Second place would probably go to "Patton".