Diablo III: Reaper of Souls

I doubt it's "Most active players". I know for a fact Towelliee hasn't touched D3 for months, hell maybe even a year and he still got in the beta.

EDIT: Kripp got into the beta and I've been watching him for most of his streaming time. Really liking what Blizz has done so far. Some of the legendary items he's gotten are freaking insane! He got the "Immunity to Cold and recover 25% of the damage absorbed" ring, but it's apparently causing his game to crash when the effect procs against Frozen rares / champions.

EDIT2: Holy pomegranate that EXP curve. 10x Paragon 100 will only put you at Paragon Level 376 with the new system.
Paragon EXP Chart: Levels 1 -> 2000