Seven is a classic twist ending. Other writer directors should study that ending.
Me, I loved the Original Friday the 13th ending. Finding out the bagged faced killer is really Jason's mother. Classic.
It would be pretty damn hard to beat "I am your father". Although I'd have to say the twist ending to "The Usual Suspects" is definitely near the top as well - when you saw that shot of Kevin Spacey's feet as his walk suddenly changed... that blew me away.
the sitxth sense bitches HELLZ YE, also that movie with robert deniro called hide and seak which had a good twist, war with jetli and jason stetham had a good twist, they arnt my all time favourite but they are good movies