Which games, from those I own already but haven't even started yet, should I play?

Guns of Icarus Online, Antichamber, Audiosurf, BioShock 2, Deadlight, Dishonored, FEZ, Jets'n'Guns Gold, Mafia II, Natural Selection 2, Spec Ops: The Line, The Darkness II, Thief Gold, XCOM: Enemy Unknown.

Spec Ops: The Line and Dota 2

Do you want to get yourself mindf**ked and enjoy it? Play Antichamber!

Spec Ops: The Line

Spec Ops The Line - incredible story

The Incredible Adventures of Van Helsing - you liked Torchlight 2 and Path of Exile, so I think you'll like this one

Blades of Time - not the greatest game in the world, but hell of a lot better than its predecessor. If you like the Tomb Raider and Prince of Persia series you'll most certainly like it

Thief - one of the greatest stealth games ever made

The Darkness 2 - this is game in which you can pull an enemy's spine through his ass. Not for everyone, sure, but if you're like me and you enjoy gory, gratuitous violence(like in Tarantino's and Rodriguez's movies), there's lots of fun to be had with this game. And like in the case of most 2k games, the story is very interesting.

Overlord - amazingly funny and fun as hell. BE EVIL!!!!!

Bioshock - one of the best games EVER MADE. Enough said. Just 0.9 hours??? WHAT ARE YOU WAITING FOR???

Left 2 Dead 2 - ONLY if you have at least one friend to play it with.

And I see you chipped in just a few hours in Dark Souls. I don't own it and I'd like to play it. You can. DO IT!

OMG stupid thread..Look on youtube...

Syberia 1 & 2 - One of the best of its genre.

Papo & Yo - Beautiful story & wonderful visuals. Rather short though.

IL 2 Sturmovik - Best WW2 flight some out there. Possible best flight sim period.

Thief Gold - And the best stealth game out there. You have a lot of "bests".