Rainbow Six: Lockdown
I know this is a pretty old game, but if you don't have it than it's totally worth the money at gamestop. The levels are fun and are all different in single player. It's really great to have a team with you the whole time even though i shot two of them in the head the first time I played. Guns are awesome too. Their all really strong and it doesn't take too much to kill the enemies. The A.I.'s aren't THAT smart, but it's not too bad. Your team is really good, and you could go through a whole mission without doing a thing, but that's no fun. There are a ton of unlockables (weapons, concept art, cheats, etc.) that you can buy with intel points that you get in the campaign. it's definitely a great single player game. I haven't tried online or co-op offline, but i heard that online is great. You can ugrade weapons and abilities and it's really good. Overall, Rainbow Six Lockdown is a game to put on the list.