TF2 for an amusing alternative to CS:GO, Half-Life2 / Episode 1 / Episode 2 for one of the best FPS franchises ever made, Broforce for indie greatness that sort of does multi-player. Far Cry possibly?
What kind of games do you like? For example, I'm on a Warhammer Dark Crusade retro-kick at the moment that would run great on your PC, not much point recommending it if you hate RTS?
TF2 for an amusing alternative to CS:GO, Half-Life2 / Episode 1 / Episode 2 for one of the best FPS franchises ever made, Broforce for indie greatness that sort of does multi-player. Far Cry possibly?