Best game: Halo: Reach. For the sheer amount of features the game offers as well as support. Worst game: Call of Duty: Modern Wafare 2. For the amount of hackers, modders, and glitchers the game has attracted and lack of support.
I recommend Fallout 3, Fallout New Vegas, Bioshock, and Red Dead Redemption. Both Fallout games are excellent, and you can get both of them for $20 each. Bioshock is also a great game, that's also $20. RDR is also an open world game that's really good, and
costs around $30.
If you're looking for arcade games, than I recommend Battlefield 1943 and Pacman CE DX.
I would tell you to stay away from Cod, but it's already too late...