Best: Aurum Vale, it just got the right blend of challenge I guess =) It can be done really quickly if you know what you're doing. And it's really the only 4-man content that actually seems to punish you for messing up.

Worts: Not too sure, I find most of them to be boring slogs that I just do for the tomes and such... Probably Xelphatol (most likely spelled that wrong), with Sohm Al HM a close second. Most dungeons post 50 doesn't really have any interesting looks or variations. And pretty much all of the mobs in them are just tank and spank.