I've found that TF2 gets my gameboner stiff since it rewards the hell out of teamwork and punishes those who try to play it like Call of Duty. There's also quite a bit of strategy involved in that you must choose the most effective loadout for your style and work with other players of complimentary classes in order to succeed.
Examples: A Heavy is basically a huge bullet magnet, ensuring he will die quickly without a Medic to back him up. Engineers are excellent defenders, but can't do much in the way of offense and rely heavily on Pyros to defend them from Spies.

