I don't much care for the urban environments, but I find Belsavis the most tedious. Corellia blew my mind with the metro system. The open-air portions of Coruscant are kinda sweet too. (Hey, I spend so much time on this game, I like to at least feel like I'm outdoors)
My main is a Marauder whom, before they fixed the healing glow bug, I played without Quinn, which meant I had to be about 5 levels above recommended to minimize frustration. Cause I hate when folks are lazy to do side quests and gear up properly, and just whine for help with their class bits. Even with Quinn healing, I had to spend about 2 days after hitting 50, running around, gearing up, getting datacrons and matrix shards, so I could solo through my class finale. Which in turn prepares you for Ilum, but I haven't done much of that yet.