Best multiplayer?

Counter strike

Hmmm, I'd go with either Chivalry or Natural Selection 2.

Planetside 2, meh its free.

Not on sale, but still fun: Shoot Many Robots (you might be able to get a cheap Steam key on Steamtrades, because this was one of the free Ubishop games)

I see you have only 6 hours on Portal 2, you should check the co-op mode and the community maps ;)

I was not aware portal was still active