It's the Void Lords for sure.
Even if we think macro-macro, the First Ones set everything up but either aren't around or aren't malicious. It's implied that they're dead anyway; the Void is the thing that, while existing in it's own reality, is seeping into all other realities and trying to overtake them. None of the other forces have the means or the intent to do that.
So even if the Light/Denathrius do show up to cause some fucking shit, the Void is the one with the real means and plan to do things. Even the Titans are pawns in whatever game Iridikron and Xala'tath are playing with the Void, and Azshara is involved in this too somehow
It's been 20 years grandpa, we're all tired of that. Also, no? I don't think anyone, ever, would argue the other faction was the actual "big-bad" in the universe, unless you're one of those brainless PvP LARPers who shout "ReD Is DeAD" and shit blood whenever anyone tries to suggest the other faction are just other players trying to enjoy the game.