This year's build of StarCraft II multiplayer was so excellent. In fact, I came away far more excited about it than Diablo III; not only because it's closer to release, but also because Blizzard's really knocking it out of the park.
The new units have transformed the game into an entirely fresh thing at this point, and nearly everything else about it is clicking in a way that I didn't even expect. The music, for example, actually lives up to the original score, which was something I did not anticipate in a million years. The beta can't come soon enough.