Good SP games

Batman Arkham Asylum and City

Max Payne 3

Metro 2033 and Last Light

BioShock games

Tomb Raider

Mass Effect games

Max Payne 1 , 2 , 3

All Bioshocks

Tomb Raider series

Most of the games I play are solely SP, so...

Batman: Arkham Asylum/City, Just Cause/2, Mirror's Edge, The Walking Dead, Portal, Orcs Must Die!/2, The Binding of Isaac, FTL: Faster Than Light, Super Hexagon, Thomas Was Alone...

I enjoyed all of those, and a ton of others, but you haven't given much indication of what sort of games you like, so this post probably isn't going to be particularly helpful.

Awesome thanks! I'm expanding my list at this moment so keep them coming :D

If you haven't got Tropico 3 or 4, go gets it! Cuz really, everyone wants to be a dictator!

Play Psychonauts, you won't regret it

Mafia 2 with Joe's Adventure+Jimmy's vendetta dlc
And Mafia 1,but that's not on steam anymore :(

S.T.A.L.K.E.R. games.

Deus exs

System shock 2

Max Paynes

Knights of the old republic 1+2

Fallouts

Vampire the masquerade: Bloodlines

Assassin's creeds

XCOM

Risens

Red factions

Dark souls

Broken swords

and others i guess :)

Far Cry 3, 2 or 1 all of them are great

The Walking Dead (Telltale's one obviously) with DLC

Black Mesa
Half-Life 2 (+ EP's)

FTL: Faster Than Light,

Star Wars: Knights of the Old Republic,

Star Wars: Knights of the Old Republic II

Crusader Kings 2

System Shock 2

Jagged Alliance 2

Vampire The Masquerade: Bloodlines

Gothic 1-3 (Get the fan patch for 3, ignore Forsaken Gods)

Majesty The Fantasy Kingdom Sim (2 is also good, but 1 is better)

The Longest Journey

Syberia 1 & 2 (stories are linked, you want to play both)

Cargo! The Quest for Gravity

Aquaria

Spec Ops the Line