All 3 games are great. However, it depends on what type of gameplay you like.
If you like Multiplayer games (Online, against other people, or cooperative), then you should get halo 3 because that's what it exceeds at.
If you like Single player games, you should get Bioshock, because it's a great single player game.
If you like both, get Gears of War, since it has both Good Single Player game, and Multiplayer. I'm not saying to get Halo 3 because if you didn't get the first 2, you won't get the story, plus it really is a bit weak in Single player, as opposed to multiplayer, which is the most fun of the 3.
All 3 games have great graphics, but if you want to rate them, then Bioshock would have the best graphics, followed by Gears of War, and then Halo 3, so if you're looking for the best looking game, that's what you should think about.
If I had the choice, I don't really know what I'd get since all the games are very fun. I'd prolly go with Halo 3, though, because of the long lasting game play, that will surely last for 3 years, and beyond.
Good luck.
