Halls of Reflection is my favorite. I remember the first couple times doing it. There was a sense of suspense about it. Running away from the Lich King, knowing if he got too close you were done for. It was almost like a old horror film. No instance made me that nervous since then.
Right now it is Grimrail Depot, the whole idea of this dungeon is great even though you don't see the train anywhere in game(whats up with that).
Ulduar is magnificent, it is definitely the best of all.
Karazhan..lots of fun!
It was a hard to choose one, but I will have to go with Warlords of Draenor's Archindoun. My first character being a Paladin, getting to protect such a holy place against the corrupt shadow magics was really fun.
The Pit of Saron was tons of fun, challenging and big, but not too big
My favorite is prob shadowfang keep
I'm a fan of Zul'Farak. Just queued into it while levelling my warrior, loved the place.
Ulduar -- and particularly the Planetarium within.
My all time favorite is definitely Ulduar. That was WoW at its best for me - pure magic; I had so much fun.
My favorite instance is probably Naxx because of the memories that I shared with my friends.
fave instance is most def ulduar. i love the size of the raid zone and the graphics in there esp for the algalon fight!
Atm my favorite instance is Grimrail Depot.
My favorite instance is Ulduar. The size/scale of that place... so much wonder. Makes me feel like a kid again.
ICC has always been, hands down, my favorite instance in the entire game. I also really enjoyed Sunwell, but... nothing matches up to ICC for me.
Karazhan, cause is my first raid.
Forge of Souls, music is just awesome