Counter Strike I'd say is still fairly popular. Not as popular as it was, say four years ago but is definitely still there. Team Fortress 2 definitely has something going for it; heck x10 has it's own TF2 server which I do run alongside with two x10 Staff Members and it populates itself many nights.
What I would do though is search for the newest game that is coming out which will support PC gaming and allow dedicated server binaries to be released. Do this and be one of the first few servers online, and you'll get your server populated in no time.