The Binding of Isaac, some what of a challenge in everyway.
By doing No Death Strike is a challenge, if you die once, you have to reset your file.
Garry's Mod. CS:GO.
GMOD and CS:GO are defently the best games there is.
Dota 2 is obv, most played game on steam
k, to lazy to type so im pasting some games from my steam library Battlefield: Bad Company 2
Counter-Strike: Global Offensive
Left 4 Dead 2
Torchlight II
Warframe
Red Orchestra 2
Killing Floor
Insurgency
Forge
PlanetSide 2
Fallout: New Vegas
Deus Ex: Human Revolution
Dark Souls: Prepare to Die Edition
Audiosurf
Sid Meier's Civilization V
Fable - The Lost Chapters
Dishonored
Tomb Raider
Natural Selection 2
Max Payne 3
Star Wars: Knights of the Old Republic
Metro: Last Light
Batman: Arkham Asylum GOTY Edition
Bastion
Tropico 3 - Steam Special Edition
Sniper Elite: Nazi Zombie Army
Ys Origin
Mafia II
State of Decay
Edit;
bleh, just look at my steam games, if i've played it and you're interested i'll give you some insight on the game http://steamcommunity.com/id/MrPallex
Tactical Intervention is quite sweet (Free btw )
Wait for christmas sales
CS1.5, CSS, and all the half life games. Old as shit but still fun to play. They even have a half life black mod that makes it look 100 times better.
Literally all I play right now is GMOD. And sometimes CS:GO.
(Free btw )