As a holy priest:
Classic = Scholo with an inexperienced or just plain bad group.
TBC = Heroic Shattered Halls. Botanica was a close second and shadow labs was fun too. OH and MGT heroic.
Wrath ... none? They were all boring.
Catacysm = Deadmines on the beta servers.
Mop = haven't done any of the new dungeons.