Tyria - North Kryta Province. I just love the beach and the Ascalonian Settlement there. I have a friend where when we have nothing to do we just go to the beach and play around (After clearing the mergoyles of course

).
Factions - Sunqua Vale. That area is so diverse, you can travel from the coastline to the mountains in only a few minutes. Plus, I just love the atmosphere. Like the secluded monestary in the mountains, or the dam at Lake Puang, or even the ministers courtyard.
Nightfall - It all sort of ran together, the entire game looked alike so it's hard to figure out one specific place I favor. Probably I'd have to say the Garden of Seborhin.
EotN - I love the area around the EotN and the Charr territory. It always takes my breath away when I come around the corner and see the Eye after coming from Boreal Station, plus I like Gwen's little garden around the corner. And the Charr lands are just so nice to be in, beautiful autumn forests, nice green grass, and such well maintained roads.
My favorite I'd have to say is North Kryta though. It's fun to go to the beach then visit the peeps at the settlement.