Worst:
Well, how far down are we talking here? SoO was fine the first few times, but I was so sick of that shit after a year and a half. There's also ToC, the standard-bearer for raids that ruined WoW.
I was also never really a fan of MC.
Best:
Karazhan, not even close. That shit was so fucking cool. By far my favorite raid.
After that, Ulduar was fantastic, BWL was interesting the first few times, and AQ40 got better as it went along IMO. ICC was all right too, I guess.