It is hard not to fall into a pattern with Steam's multiplayer games (for me, personally). Counter-Strike: Source and Global Offensive, Left 4 Dead 1 and 2, and Team Fortress 2 seem to be the go-to games for multi-player mayhem.
I think that DOTA 2 is good, but takes some commitment to learn: it is not a game that a group can easily jump into without everyone having played it before. Alien Swarm is also pretty good for having been a free download. Sanctum is pretty fun for a 4-player game. I was never able to get a group of people to play Torchlight 2 together.
Thought it doesn't have cloud support, Unreal Tournament III: Black edition is one of my favorite FPS deathmatch games.