Any Quake game is a good call. They are just basic FPS games done very well, none of this fancy faffing with special abilities/powers and other nonsense.
Quake 2 will be very hard to get hold of because of it’s age, but it’s a classic. 3 is good if you’re thinking of multiplayer later, and 4 is just a good scare/shoot fest that can either be very easy or almost impossible depending on difficulty setting.
Or the original Half-Life. Awesome game. If you can be arsed with the faff of Steam and it’s pointless, CPU consuming stupidity and piracy paranoia then get it with the source engine or even have a go at Half Life 2. Half Life 1 does show it’s age, but it’s still good fun.