I think a better question would be who would win in a FFA cage match with all the classic hero's. But just the singular, not a team. He-Man, Liono, Gi-Joe, 1 of the turtles...fuck it...throw in a power ranger just to watch them die... I could keep going here.
It'd be an epic battle but even assuming the same number of He-Heroes to Thundercats, it's still the Masters. How big could this fight get? Will the Turtles turn up? The Silverhawks? Jayce and his Wheeled Warriors? Damn, probably not since they'd all get crushed.
@Super-Munkyboy: a Skeletor/Mumm-Ra battle would be good, although I imagine a little dusty. A single fire would probably finish both the crypt-living villains. But you're right, a with a weakness that most women carry in their handbags, you'd be worried.