I prefer open world games. Currently playing Acandia Gothic 4. It's not as open as I would like but it's pretty fun and quite beautiful in places. I love the Fallout games but haven't finished New Vegas yet. I was hoping the game would be much, much bigger than the last one, but it's not.
As far as graphics go, Just Cause 2 is the best game out there. Not only is this game insanely huge, but it's incredibly realistic looking. I would say the play area of the game is at least the size of a typical US county. Even in a fighter jet, it's takes a bit of time to fly across the world, let alone around it. The amazing thing is, there is no lag or wait time, other than when you die or call in a transportation ride.
The downside is that the game play sucks. There are some fun missions but 99% of what you do is just the same thing over and over again. You can beat the game and see only about 10% of the world. It would, however, make the perfect world for the next Elder Scrolls game!
For me it was Motor city Online. It was the best racing game I've ever played. I believe it was the first game to be subscription based. The biggest problem with it was the fact that it took a long time to be a good played and it was very hard to make money at the beginning and that drove a lot of the new players out of the game.