Some candidates:
Planet of the Apes. Yes, I know, it's a cliche and a punchline now, but the final shot of that movie shocked and surprised audiences and it's still one of the best movie endings ever.
The Godfather. The last 10 minutes are amazing, because it is all about the characters: Michael's rise to power, realized… And how everyone reacts to what he does.
Citizen Kane. What is the measure of a man? Sometimes it's what he's lost and can never get back that makes him who he is. The final reveal of what Rosebud is, and what it represented, is one of the great endings. Ashes to ashes, dust to dust.
The Usual Suspects. Who is Kyser Soze? Again, a mystery revealed brings a great resolution to a great film.
One Flew Over the Cookoo's Nest. Chief's escape from the institution is one of the most wonderful moments of cinema.
It's a Wonderful Life. George Bailey, finally appreciating the whole of his life – warts and all – running through the streets wishing everyone "Merry Christmas!" – well, it don't get any better.
Chinatown. The best unhappy ending, ever. The bad guys win… As they so often do.
Vertigo. Hmm, another Jimmy Stewart performance. This time, Stewart is the opposite of where he was when he played George Bailey. Here he is apoplectic and obsessed and… Well, no spoilers, just watch the movie.