What movies are most ruined by spoilers?

I know most movies are actually better if you go in not knowing too much about them, but some are totally ruined if you know the key mechanic or plot twist in advance - i.e. The Sixth Sense, Memento, etc. I just watched Fight Club for the first time tonight (I know, I'm way late to the party), and since I already knew the big twist, it wasn't as fun as I was hoping it would be. So I was thinking...

What else should I watch before it gets spoiled by spoilers?