Best group instance : Xibaluku, it's less polished that the more recent dungeons like the superb sepulcher, but the atmosphere in there is very appealing to me.
Worst group instance : Black Castle, it feels empty and meaningless, and it's totally out of proportions, it hurt my eyes each time i go there.
Best raid encounter : Zodiac, even though i always lag like crazy in there i just love the design. If only Funcom could sort the terrible boss pathing and the buggy symbols. Plus, the room itself is a masterpiece.
Worst raid encounter : can't say there's any i don't like, they're all cool from a certain point of view.