in global ulduar ofcourse..being able to clear an instance 4 times per week (totc) is just crap..
icecrown kinda takes away the "achievement feeling" of killing a boss.. seeing how you gotta do him on heroic aswell..
Also the 10/25 man raiding is a bit lame but ok..
Imo the best hard mode is XT2(something) deconstructor or whats he called..
Activating a hard mode by doing high dps is the best idea ever..
all in all... TBC was the best ... no hard modes required cause raids where hard enough already..