Left4Dead 2 more than 11 million owners
HalfLife 2 more than 8 million owners
Skyrim more than 8 million owners
Civilization V more than 7 million owners
Portal 2 more than 6 million owners
Borderlands 2 more than 4 million owners
CoD Modern Warfare 2 more than 4 million owners
Grand Theft Auto IV more than 3 million owners
Fallout New Vegas more than 3 million owners
Tomb Raider (2013) more than 3 million owners

Deponia: Close to 380.000 owners (would you have thought? I'm surprised and impressed); Chaos on D: 138.000, Goodbye D: 77.700

The Book of Unwritten Tales 2: 9.500 (disappointing start); BOUT 1: 169.000, Critter Chronicles: 64.000

A New Beginning: 184.000

The Whispered World: 150.000

The Night of the Rabbit: 72.000

Chains of Satinav: 143.000, Memoria: 67.000

Edna & Harvey: 69.000, Harvey's New Eyes: 162.000

Broken Age: 241.000

Broken Sword 5: 91.000

Grim Fandango Remastered: 74.500

The Secret of Monkey Island SE: 450.000

Monkey Island 2 SE: 346.000

indiana Jones and the Fate of Atlantis: 103.000 (wow, they really should consider a Special Edition à la  Monkey Island)

Dreamfall Chapters: 69.000, The Longest Journey: 182.000, Dreamfall TLJ: 111.000

The Testament of Sherlock Holmes: 242.000

Machinarium: 670.000

Botanicula: 349.000

Paradox' recent releases:

Cities Skylines: 900.000

Pillars of Eternity: 246.000 already

Hehe, I find it funny that Minecraft has sold a lot better than the best selling Steam game, even when counting F2P games.