#17 - Team Fortress 2 (Kudos to no-one for guessing this)
In a generation where I thought I was done with first-person shooters because they didn't cater to me anymore, TF2 came along and reminded me that the genre can be massive amounts of fun if it doesn't take itself too seriously.
The fact that you have 9 different classes to play as meant that this has been one of the longest-running online FPS games I've ever played (iirc, 300+ hours and counting). I keep coming back to rocket jump around as a soldier; stab people in the back as a spy and build an impregnable fortress as an engineer. THe variety in maps is fantastic and - as I said in my clue - they keep adding content even half a decade after release. The new mode is horde mode and it's absolutely heaps of fun.
Oh, and the "meet the" series introducing the characters was literally sublime:
