BEST NEW LEVEL: Taejin's Tower (FFXIII)
SSFIV's Solar Eclipse and Crumbling Laboratory were fun, but I can't really give a KB GotY award to a background. Siren's Alley in BioShock 2 really stood out as well, using a relatively small space to convey a great deal of action and narrative, but it was not as impressive as the Tower.
Taejin's Tower was one of many visually stunning environments in Final Fantasy XIII, along with Orphan's Cradle, Oerba, and Gran Pulse in general, and it was one of the relatively few with bearing on gameplay. It is actually a little talked about series hallmark, joining the ranks of FFVII's Shinra Tower, FFVIII's Deep Sea Research Center, and FFX-2's Via Infinito, among others, as vertically oriented dungeons with enumerated levels.
While not labyrnthine, Taejin's Tower contains some of the more robust puzzle elements in all of FFXIII, which isn't really saying much.
(I really would have liked to have had LoL's Magma Chamber up for the award, but , *cough* riot *cough*.