Best Co-op shooter ever made. Infinite replayability and almost two decades of modded content easily accessible through the workshop. It still has an average of almost 30,000 people playing at any given time. Plus, Valve isn't overly greedy so they actually sell it at an appropriate price for its age. It is worth ten dollars. When it ends up being 5 dollars in the future, it will be worth that.

The thing that makes this game different is its allowance of personal skill. Most cooperative titles try to force team-play through handicaps. Left 4 Dead 2 instead is designed such that a single player can beat every campaign if they're good enough, but mistakes are punished heavily and you need your teammates to assist if you get grabbed or downed. There are ways to mitigate even that, if your aim is good and you shove at the right moment.

It is the ultimate example of how to do cooperative and asymmetric multiplayer, without a single game inspired by it actually understanding why it works. Much like Half-Life, the original is fantastic, it's derivatives are usually hot garbage except for some obscure indie titles made by small or single man teams.