Portal. Hands down.
On another note, game endings usually sucks. Like completely. They never really feel like a conclusion, partly, I guess, because you can't play them. The usual feeling I get when watching them is 'YES! I beat the game!', which is a good feeling, but it's not really a good feeling when you are watching an ending sequence... which is why the Portal ending was so perfect. It's the first game I EVER played twice just to get to the end.
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