I've always enjoyed Poet's Palace within the Shimmering Citadel. The mobs are in their low 60's, and there are plenty of nameds placed evenly around the area. I also like the puzzles and quests within the instance.
Back when I used to do this, there was an access quest that at least one person had to have completed in order to enter. I don't know if this is still the case. If it is, you can visit Kedar the Curator in the Shimmering Citadel to start the access quest.