Well, for the consoles, I'd have to say Shadow of the Colossus. With most of the boss fights, I felt I would actually die if I fell off the things. You can go on and on about draw distance and the like, but when falling off of a 6 story high behemoth feels just like that, the game's doing something right.
PC-wise, I'd have to say the Waverider map on Combat Arms. There's some nice buildings and such to run around on, you can lounge by the ocean, and look over nearly the entire map from a nice tower. Shortly before you get owned by someone who was actually playing the game.