Before Cata I used to keep my Alliance toons in Exodar and my horde toons in Undercity. Now it feels like you're gibbing yourself if you don't hang out in Org/SW for the professions dailies (particularly the JC daily), the PvE/PvP vendors and the portals. Definitely like the Undercity most. Sylvanas for Warchief in 5.0!
I like Ironforge for its great accessibility. Once you learn the layout, moving around is very swift. The only real fart is the size of doors and hallways...
Dalaran's also one of my favourites, even with the occasional FPS problems back in the day. Book hunting, fountain fishing, the streetlight gnome, mammoth racing, the wine and cheese daily... Dala had it all, and I'm sad to see it go.