Team Fortress 2 (PC, 2011)
This, to my mind, is the best online shooter ever made. No arguments. You choose one of two teams then one of nine classes, each with its own traits, health, perks and weaknesses. For example, the spy can go invisible and kill any class with a back stab. But the invisibility is limited, experienced players are very wary and Pyros will destroy you completely due to their fire based weaponry and your lack of health. Unlike so many other games, there are very few ways to be cheap (besides maybe sentry guns and spawn camping) and the top players are very skilled. This game was my graduation from cute little fun single-player games into the blood-and-guts world of online gaming.
Fun fact: I have spent 560 hours in 4 years, or a combined total of 23.3 days playing this game, making it my most played game ever.