Judging the encounters and environment only, Icecrown hands down - however I enjoyed the people in my guild and raid group the most during Karazhan and later on Naxxramas. Haven't been in a good guild since late Wotlk, can't judge Cata content fair.
Just get the fuck after it, don't be a pussy, and you'll be fine.
in this world full of people there's only one
and if we only live once, I wanna live with you