My main game to play on Steam is Counter-Strike: Global Offensive. It's by far my most played game with over 760 hours, it's competitive feel and the 5v5 format helps the game stay fresh and fun during each and every game! Definitely a recommendation from me. My next most-played game is ArmA 3, however I don't play that much anymore. ArmA 3 is great fun if you get a group of guys, about 10 or so, to play king of the hill or RP servers with. For some people, military simulation groups are also a main reason to play the game, but after being in one for 6 months I realized it wasn't for me. Other than that, my steam library revolves entirely around games that my friends are playing and whether or not I'm good enough to play with them haha.