Got to be Fight Club and se7en, he is also particularly brilliant in Snatch.
I was working with a guy once who told me he was on holiday somewhere, I can't remember where but it was the filming location for Troy, he was in a bar or hotel or something, and there was pictures of all the cast up on the walls, like head shots but also pictures of the cast and crew all drinking and posing with locals and the staff who worked there. He noticed Brad Pitt wasn't in any of the pictures, not even a head shot, he asked the staff why this was and they replied it's because he's a complete jerk. Apparently he refused to associate with any of the staff in a manner of they were beneath him, and he was constantly rude to them, they hated him.
Might not be true, might be, who knows, who cares I suppose, just thought I would share that tale, still think he's a great actor, even if he might be a bit of a d**k.