I liked Maraudon, simply because the dungeon itself looks great and has a nice feel to it. All the large rooms and lots of water is great; I really enjoy the scenery.
Well, bear in mind i've only been through two total instances. I would say my favorite was Wailing Caverns. Mainly bc its the only instance i've done by myself with no help from lvl 60s. I also like the creepy feeling of going through there.