Team Fortress 2
The real excitement in this game comes out of the many game modes presented to players. Each game mode brings a new feeling or possible set of tactics to the table without ever letting players get lazy. The goals often require a lot of teamwork and some self-coordination, but never ever seem to crazy or too out of hand to be fun. The achievements are also a fun touch, with there being 518 of them at the time this was written, and each of them pushing the player to spend some time improving themselves... or doing something utterly ridiculous (and often hilarious). Overall, this game is a great time-waster, and I definitely recommend it to anybody with a Steam account and some time to burn.
The game can be found here (as of 6/22/2016): http://store.steampowered.com/app/440/
The extensive backstory in comic form can be found here (as of 6/22/2016): http://www.teamfortress.com/comics.php#