Your Favourite Videogame Environment?

The past couple of weeks I've been thinking a lot about game environments that do something a bit different, or game environments that I really love spending time in because they are singular and unique (Hello, The Zone!).I wanted to ask you guys, too. So what game environments do you keep coming back to? Which ones took your breath away with their cleverness? What are the most interesting game environments, and why?

Speak!