Favorite Game in Steam?

Angry Video Game Nerd Adventures.

I haven't really played much of anything else on Steam, mostly old games I used to play as a kid that were originally made on DOS or Windows 95, and would never run on 7 or 8 today without the Steam treatment. Commander Keen 4 is another favorite.

But yeah, surprisingly AVGN Adventures is just that good. It does have challenge, I love the graphics and sounds, controls are easy to get used to - I can even use Joy2Key, which allows me to use a SNES controller with the game (I have USB ports for SNES and NES controllers).