Probably ICC for me, followed by kara and BT.
I really do not get this nostalgia boner people have for ulduar, the raid was kinda junky. The only thing it had going for it was the unique methods of activating hard mode. The bosses themselves were tedious and friggin annoying, a lot of them had insane amounts of downtime (looking at you mimibore) and the tier of gear there was one of the must uninspired tiers in existence for all but mages and death knights.
Though no raid will be worse than highmaul IMO. My god that place was dull.