While Once Upon A Time In The West has to be up there, its too referential to other films to be perfect.
I love For A Few Dollars More, with the tiny figure on horseback being watched by a bored, whistling, off-camera sniper (actually Leone himself)
then, as he's finally shot, you hear the springy 'boing, boing, boing' of a jews harp announcing the beginning of the fantastically unique Morricone score. The mad credits and the gunman lobbing an unaimed shot at the bucking, riderless horse show Leone's playful humour.
It always makes me smile no matter how many times I see it.