TBC: Magisters' Terrace (amazing music, love the Blood Elf theme)
WotLK: Culling of Stratholme (always wanted to see Stratholme before the Scourge ruined it)
Cataclysm: Vortex Pinnacle (really well designed instance.) Loved Well of Eternity too but it should have been a raid
MoP: Temple of the Jade Serpent (has great music and theme)