Counter Strike: Global Offensive
Where to start with this game? First of all, it's priced cheaper than most of the AAA FPS titles and will give you just as much, if not more, enjoyment as those, and during steam sales, the price will drop even more. From the large variety of gamemodes to the perfectley balanced weapons, and the mods that range from realistic to complete insanity, this is well worth the 15 dollar [or cheaper] price of entrance.