Black Temple
Ulduar
ICC
The one I've had the most fond memories of was BWD. It's mostly due to the fact that I raided with a lot of friends and fun people who didn't really care too much about being "cutting edge" or anything - just about having fun in the raid. The raid itself is a masterpiece too, great visuals, and not a single boring boss. Plus, of course, Nefarian chatting around.
Judging by the quality alone tho, probably Ulduar, maybe for different reasons than most people. I keep hearing that the highlights were the "ultrahard encounters", like Mimi hardmode or Yogg, but for me it was all about what made the raid different - starting in a giant vehicle derby, fighting a giant whose body you use as a bridge, messing around with different Council strats. Even the trash was fun.