Good to know there are other gamers on here as well!
I don't do much online gaming though, I'm old school in that I find single player stories more appealing than playing with a bunch of random strangers. I never buy a game for its online mode, I do however make exceptions when it comes to Elder Scrolls Online, Team Fortress 2, and Grand Theft Auto 5. I'd be more willing to play online if I had more friends owning the same titles as me.
My current collection off the top of my head:
Thief 1, 2, 3, & 4 (Thief 1 & 2 are my all-time favorite games)
Bioshock 1, 2 and Infinite
Fallout 3 & New Vegas
System Shock 2
Assassin's Creed 2
Dishonored
Left 4 Dead 2
The new Tomb Raider
Half Life 1 & 2
Portal 1 & 2
Red Dead Redemption
Far Cry 3
Battlefield 3
A ton of Call of Duty games
Crysis 2 & 3
Grand Theft Auto 4 & 5
Wolfenstein The New Order (Currently playing)
Outlast
Civilization 5
Team Fortress 2
Sir, You Are Being Hunted (contains pipe-smoking Victorian killer robots)
Elder Scrolls Morrowind, Oblivion, & Skyrim
Elder Scrolls Online
Kerbal Space Program
Age of Empires 1, 2, & 3