Which game should I play?

Woodle Tree Adventures

Deus Ex: Human Revolution

If you like Saints Row you may like Just Cause as well. :)

Nuclear Throne and Oddworld: Abe's Oddysee

play dat half life 2

Magicka

torchlight.

Undertale :3

Metro 2033 and Metro Last Light

tetris

darksiders / magicka / witcher

Magicka (better with friends)
Deus Ex HR (but IMHO it'd be better to play the previous chapters first)
Hitman 2

System shock should be good also, if you like cyberpunk.

I liked Kane and Lynch: Dead Men, but it doesnt have good reviews

Dragon Age: Origins

Hitman
Forced ( With friends this game is awesome)
Half Life 2
Dragon Age: Origins

Tropico 5.

Killing Floor

Dragon Age Origins! Now! Immediately!
If you have 3 friends: Alien Swarm. Free game with veeeery awesome moments :D Can't be played alone, sadly :<