Well of Eternity would be my favorite dungeon of all. Reason: I just -love- the lore of the Great Sundering and everything it brought with it and how the world was changed with it. And the fact we can go back in time and actually live through that little piece of lore.
Karazhan. It's fun to explore and if I could, I'd move in!
Karazhan, reminds me of the good ole' days.
Icecrown Citadel :)
My favorite instance would have to be... Icecrown Citadel. This is my favorite instance because of the lore behind it, with the attempted assault by Illidan Stormrage and his crew, and just the overall lore behind Arthas Menethil and the Lich King. Plus the gear from it looks sweet and really fits each classes personality in a way... Another one I really enjoy is Black Temple also because of the lore behind Illidan Stormrage and the Night Elves... I'm a huge lore nerd so any instance that plays, or has played a big role in WoW history is exciting and awesome to me
I really used to love Zul'aman for the timed run, as well as Karazhan. I am just now coming back since Lich King, so looking forward to see all of the new dungeons between here and there.
My favourite instance would have to be one of my first: Throne Of ThunderHorridon was mindblowing at the time, just in terms of his size!
Halls of Reflection
My favourite is Scholomance and the edgy goth kid inside me approves.
My favorite instance is Black Temple!
I'll always have a soft spot for the original Deadmines. It may date me, but I still remember spamming LFG chat for a healer or a tank when leveling through Westfall. :)
Definitely new Auchindoun. Demons, flying giant shields, space goats - what's not to love?
My favorite instance is a toss up between Classic Zul'Gurub and Stratholme.
I've got a fondness for The Arcatraz, and I've never been sure why.
It has to be Karazhan! It was the first raid I completed with my guild as a whole.
I'd have to say the Firelands...:3
My favorite run is Kara, especially the theater.
If we're talking instanced areas I would say the farm in pandaria has to be my favourite both as the first "playerhousing" and as a mood-setter for the whole continent.If it has to be a dungeon then old Maraudon was fantastically athmospherically and lore-wise. It was very underrated because of the complicated layout and entrance. But it had some of the better fights and funny trash. It was also for many the first time a resistance bonus would become important back when that was a thing. Basically it was a little raid in and of itself but for low-level 5-man groups.