I haven't had the attention span to play single player games in a minute but some that come to mind are:
bioshock: cool, creepy aesthetic. interesting gameplay, interesting story+characters
metro 2033: similar aesthetic but extremely immersive. Recommend turning the hud off
arkham asylum + arkham city: cool batman games
hotline miami: top down shooter. fun, 'repetitive' type of gameplay
Overlord II: fun casual fantasy rpg where you play the bad guy (can safely skip the first game)
Darksiders: cool beat em up, think god of war
Mirror's Edge: parkour, unique gameplay
Dead Space 1 + 2: spooky alien spaceship horror games
Super Meat Boy: fun, challenging platformer