Left 4 Dead 2
2 days 6 hours
1 day 21 hours
Counter-Strike: Global Offensive
Left 4 Dead
2 days 1 hour
S.K.I.L.L. - Special Force 2 (Shooter)
Can confirm Fistful of Frags is great fun. Adding to the list: the new Unreal Tournament, No More Room In Hell, Halo Online (search for it on reddit, it's basically Halo 3 on PC).
I've never tried: Brawlhalla, Tribes: Ascend, Dirty Bomb or Loadout
Dirty Bomb and Loadout are really cool, you should try them!