Not to resurrect a dying thread, but ICC was actually my favorite. I just really liked the theme of it, as well as the bosses. The fact that it was a huge effing fortress in the middle of Icecrown was just so awesome. The graphic designers of Blizzard should be proud of their work, because it was very nice looking. Getting through the first ten bosses was an achievement by itself, but Sindragosa and of course Arthus were just magnificent fights in my opinion.
I also enjoyed Ulduar, but that was in my nubby days when I wasn't exactly versed enough in raiding to enjoy it.