Zul'Gurub has always been my favorite
My favorite Instance is the Well of Eternity from Cataclysm. Mostly because of the lore depicted in it.
Boring I know, but it's still Karazhan :)
Ulduar is without a doubt my favorite instance. It was a great success of an experiment on blizzard's part into the use of vehicles as well as hard mode mechanics, and also had some of the most fun--as well as hardest--encounters in all of WoW's history, such as firefighter Mimiron.
I love the design and atmosphere of The Vortex Pinnacle
Since i guess "instance" also includes raids (since they're instanced^^), i definetly go with Ulduar! Loved that raid, pretty much started with it as a guild.
the original Zul'gurub was my favourite instance, they ruined it!
icecrown citadel love the Rig , have had one for quite a while --about 1.5 years I think --excellent headset
It's so hard to pick just one! I really enjoyed End Time back in Cataclysm.
I don't care what people say: Blackrock Depths FTW! I have my best memories as a newbie from that instance, including a break to hang around at the Grim Guzzler. Dungeons aren't like that anymore =(
Violet Hold for sure
My favorite instance is Molten Core. I have great memories of 40 man raids, on dial up! :)
Pre-change Zul'Gurub. Loved the distinct Loa sections and all the minibosses, even if it was near impossible to persuade people to do them (especially the Madness bosses). The mounts that dropped were always a plus as well.
Love fire everywhere, so Firelands.
Favorite instance? Definitely Karazhan with all that music, those were good times
I'm not entering, but I am really happy they used the bnet launcher to feature something like this. Its absolutely perfect and they seem to be taking to heart the outcry of people telling them to use the damn launcher for stuff like this. GG Blizz, you're restoring some of my lost faith in you.
My favorite instance that gives me the most memories definitely has to be Mount Hyjal, I know people are gonna be like this guy is a idiot or something but no. Mount Hyjal was when I made the biggest mistake of not reasearching a fight when I was a total noob way back when and asked like the number 1 guild on my server if you could kite archimonde into the water to silence him lol.
Deadmines, I can still remember the first time I ran that place and seeing the giant ship at the end.