Diablo 2, probably, as it's supported to this day.
Also, TF2 has been available on Linux for quite a while now. Please update your charts accordingly.
From a "we left the servers up" perspective, there are games older than diablo II still running. Off the top of my head, everquest and ultima online are both older than Diablo 2. What I meant was new content releases, and of course bug fixes, etc.
Come to think of it, I think UO had a new content patch a couple years ago.