ZH or CnC 3 would be fun. I'm better at ZH but either would be fun to get other folks to play against hah. Left 4 Dead tourney sounds kinda meh, I'd rather it was a proper multiplayer fragfest like Quake3 style, or Battlefield or something...hmm. UT2k4? Hah! I'm not very good at twitch games any more though....
maybe even a BF3 tourney of some sort, depending on how hard it would be to find an empty server to host one on or something like that....? Though it would be more of team thing than a single person so Im not sure that's a good idea.
We have a Quake Live and Unreal Tournament 3 already in place for the LAN. The Quake Live tournament is 1v1 and the Unreal Tournament 3 is FFA, so that should be fun. If you are into Starcraft 2 then we also have a Starcraft 2 1v1 tournament happening, we should start sign ups sometime tonight or tomorrow.
Edit - Tournaments can be found here