I say Cobra. Nigga had the whole military after him.
The rest only required a squad at best.
Skeletor and mumm-ra were powerful, but i felt like it was only a regional threat at best. And the Thundercats were like a 6 man squad that had him in check.
Skeletor was treated more like a dragon threatening a village. And still he-man and a few others kept him in check
But cobra ran the largest terrorist cell on the planet, and was routinely executing world domination plans that required them to keep pulling the deepest and most secret ops type niggaz on the assignment.
Like every extra highly trained nigga in their whole universe had to eventually get called up to deal with Cobra. And this nigga kept coming back.