I really like Undercity! It's super compact, so it takes no time to get from one point to another and you can sit on your mount all day everyday. The only bad thing about it is, that the zeppelin to Grom'Gol takes forever to arrive...
Thunder Bluff. Open area, pretty atmosphere, calming music, easy to find most things.
Undercity is a rotten sewer and tough to navigate, Orgrimmar is too big for me, Stormwind is confusing to find places, Ironforge is okay, and so is Darnassus.
Want your game to look visually stunning and run silky smooth? Read my guide. Shadowlurk - Level 29TrollRogue(Expert Leatherworker / Expert Skinner)