Wrothgar.
The story was good (it didn't have the daedra du jour behind it), public dungeons done right - still among the best non-instanced places to grind XP to this day , a lot of thought and care went into the geography and landscapes, memorable [to me] side quests and characters, challenging WBs (one of them really tricky to solo without the devs giving it cheesy 1-shot mechanics).
All in all the expansion I am most fond of and one that if it wasn't for the whole Maelstrom Arena weapon fiasco would be untarnished.