Beauclair, by far, I loved everything about it, the architecture was great, as were the colors, the music that can be heard on its streets was very fitting, the people out there were likable (especially after Geralt won the tourney).
I also loved Vizima in the first game, especially the Temple Quarter.
Novigrad was alright, I guess, it was nicely designed but the most boring chapter of TW3 is happening in there, maybe that's the reason I'm not liking it more.
I didn't really like Vergen, it felt like too much like a labyrinth to me.The music was phenomenal, though...