I know I'm supposed to pick just one, but that's not possible I'm afraid
For vanilla it's a tie between Blackrock Depths and Blackrock Spire, followed by Stratholme and Scholomance. Those were just massive and felt like real adventure.
For TBC, hands down Durnholde, followed by Arcatraz. First because of all fun stuff in there, references, characters etc, second for looks, fun bosses and generally nice experience.
For Wrath, it's hard. I loved every single one of those :P But lets say Halls of Reflection. But really, I think they all looked and felt amazing.
For Cataclysm, Grim Batol and Vortex Pinnacle (they looked incredible!). Loved trio of 4.3 heroics too, especially Well of Eternity.
For MoP, Shado-pan Monastery and Mogu'shan Palace. Again, for great looks
As you see I mostly care for looks and feel of instance. It stays even long after it's outdated. I still find myself soloing those for fun every now and then.