What are some great single player games?

Blazblue Calamity Trigger is a masterpiece, one of the best Fighting games ever made, plus it has a great story.
Postal 2 is the definitive pissing simulator, you can do it anywhere you want, it's great.
Serious sam 1-3 are great, can't say anything for all the spin offs, but the main series is definitely enjoyable.
If you liked Bioshock you'll like System Shock 1 and 2.
Try out the elder scrolls and Fallout games, I enjoyed them quite a bit.
Finally, I'll throw in Recettear because I love it.

Rogue trooper, The Witcher, Deus Ex, Metro 2033, Half Life are best. And all games above :D

fallout 1234 Dragon age 1, ME 1-2 and somewhat 3, Final fantasy games, TES series ... plus there are always Football maanger/nba games which can be played, solo, for...ever?

Half Life series
Best game ever played in my life

Dust: An Elysian Tail, DmC Devil may Cry, Limbo, Mark of The Ninja, Mechanic Scape, Need for Speed Hot Pursuit, One Finger Death Punch, Outlast, Plants vs Zombies, Resident Evil Revelations, Hell Yeah!

Avencast.

Max Payne series(even the third one), Spec Ops The Line, Bioshock and Bioshock Infinite, S.T.A.L.K.E.R - Shadow of Chernobyl, F.E.A.R., Return to Castle Wolfenstein, Half-Life 2, Deus Ex and Human Revolution, Prince of Persia Sand of Time trilogy, Medal of Honor Allied Assault, Call of Duty 2, Red Alert 2, World in Conflict, Neverwinter Nights, Star Wars Jedi Knight: Jedi Academy, Dishonored, Thief series, Splinter Cell Chaos Theory, Need for Speed Underground 2 and Most Wanted(THE ORIGINAL), Grand Theft Auto 4, Prototype, Spider-Man Web of Shadows, Red Faction, The Saboteur, The Suffering and Ties That Bind, Unreal 2 The Awakening.