Defense of the Ancients 2 by far...
I have been seriously addicted to this game for the past few months. Like the kind of addicted to a video game I haven't been in a very long time. I even listen/watch streams of it while I am at work.
Technically it's still in beta, which has been going on since the beginning of the year. But it's so easy to get an invite that I still consider it a release.