Probably a tie between Black Temple and ICC :P
It may sound silly for someone who started playing in MoP, but favorite instance would be Molten Core. It was the first raid I did, and I was terrified. "Boss-level enemies? What?" I was incredibly, probably excessively, proud of myself for soloing it.
Ulduar was the best!
Icecrown Citadel for sure
Karazhan. It was my very first instance and still my favorite. Both dragons were awesome to fight but my heart belongs to Opera. I was usually the designated victim to die so we could discover what scene it was this week. Would it be Red Riding Hood? Romulo and Julianne? or Wizard of Oz?
My favorite instance is Grimrail Depot, 'cause fights in a train are cool =)
Karazhan is my favorite, with Zul'Aman a close second.
Karazhan is my favorite - it's the best creepy castle ever! Ghosts and evil wizards and demons everywhere! Hidden passages and shortcuts, a whole opera house? It's simply the best.
Razorfen Kraul through TBC
My favorite instance is Sunwell because Kil'jaeden is my favorite raid boss :)
I'd probably have to say Karazhan, as it was my first serious attempt at raiding after dabbling a bit in vanilla.
Dungeon: The Wailing Caverns! Have some fond memories getting lost in there lol :)Raid: Karazhan! Killing each boss felt like a real accomplishment at the time
The Black Temple
I am a MoP baby so I have to go with Stormstout Brewery!
My favorite instance in WoW is stormstout brewery ! I love it so funny :)
Naxxramas has to be my all time favorite instance.
The old Dead Mines from yesteryear was my favorite instance, B4 it was redone. That was just plain fun.
I always liked the culling of stratholme personally
The old ZG.