I cried just reading you guys' responses...geeze.
Here are my biggest tear-jerkers, although I've become immune to most of them now:
Moulin Rouge - The death of Satine and Christian's subsequent emotional breakdown.
Revenge of the Sith - Order 66, and the last 20 minutes of the film. So much emotion in those scenes.
Up - The opening scene where the man and his wife grow old together, then she dies.
Titanic - Some of the sinking scenes are hauntingly beautiful, but the very last scene where Rose goes back to Titanic and sees all those who died, like her maid, the band, and Jack waiting for her at the clock always gets me.
The Family Stone - Many scenes in this movie get me going, particularly when it is revealed that the mom is sick.
Milk - The scene after Milk's assassination where his ex-boyfriend Scott and friend Anne come across thousands of mourners in a candlelight vigil. It's the hardest I think I've ever cried in a movie theatre. I made quite a scene.
A Walk To Remember - When Landon asks Jamie to marry him, despite the fact that she's dying of cancer. And then the rest of the movie.
Half-Blood Prince - Dumbledore's mourning/wand raising scene. The score there was magnificent.
I'm sure I'm missing some, but those are the ones I can think of off the top of my head.
Spoiler: 
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