I played some 200+ hours of Brink. They advertised a cooperative campaign mode that I didn’t realize was just going to be the multiplayer mode with bots on one side. I wanted very badly to force that game to be a new Left 4 Dead, but all it was ever going to be was Team Fortress 2.

Left 4 Dead 2.
Maybe not the oldest (2009), but as a Source Engine game it certainly feels that way. I keep looking for something to replace it in my multiplayer game roster, but so far, nothing has managed to retire it completely.