Here's my personal reccomendations:
-Just Cause 2
Comments: Best action game I've ever played. ONLY PROBLEM: controls take some time to get used to, especially in the PC mode, it's a pain.
-Call of Duty Modern Warfare 2
Comments: Amazing single player experience aswell some good stuff in multiplayer.
-Half Life Series
Comments: Amazing Single Player and Story. HALF LIFE 2 AND IT'S EPISODICS: voice acting is done incredibly good and the graphics are really good for it's time (2004)
-Need for Speed Most Wanted
Comments: Best arcade-styled racing game. It's very fun but it took me forever to finish it.
-Team Fortress 2
Comments: Class-based multi player first person shooter action at it's best.
-Left 4 Dead
Comments: 4-player Co-op first person survival horror game. One of the best of it's kind.
-Left 4 Dead 2
Comments: Same as L4D but much more improved.
-Killing Floor
Comments: Co-op first person survival horror game. It's very cheap price truly is worth it's experience.
-Beat Hazard
Comments: A space shooter where you shoot enemy spaceships according to your music's beat. Incredible experience but the visual feedback in it is so large you may go insane before you get a seizure out of it.
-StarLancer
Comments: The best spaceship war simulator I've ever played yet. It's very addicting and extremely fun if you have a Joystick. It's ALMOST the best game I've ever played. I've beat it three times when I was a kid and it has 2 endings.*
-Hellbender
Comments: Very old and may be buggy with modern gamepads and systems and you must use a tweaky code in Command Prompt to install it in Windows XP and up, but it's a very entertaining space shooter.*
* : Includes Multiplayer but you must use Hamachi, as there is no other way as servers are non-existant due to the time being.