Skyrim's up there, definitely. A large world with some very evocative locations. There are, however, some areas which are a bit on the dull side.

Assassin's Creed has come up a fair bit in this thread, and now I'm going to throw in my contender: Istanbul, from Revelations. Absolutely stunning.

Borderlands 2 is also up there for me. Much more variety of environments than in the first game, and some great vistas. Some areas have a lot more character than others though, it has to be said: some are completely forgettable.