hmm ok,
pre-tbc
Deadmines =), Scarlet Monastery(dno just love the place), Uldaman(nice looks/lore), Sunken Temple and Scholomance cuz of the creepieness and zulfarrak is nice as well, always been fun to run it on an alt =D and Maraudons looks are also rly cool
<3 pre-tbc instances cuz of the nostalgia ofc, lots of different cool themes, and because some of them are actually "long", dont like to rush through an instance ;/
BC- Magisters Terrace i guess
WotLK- pfffff Oculus, unique and at least "hard"(kinda=), and then i'll have to go for Draktharon and Gundrak because of the Trolls =)...guess that says enough...