Each and everyone of 'em!
Each one has it's own uniqueness that brings a certain element to the MOTUVerse! Who's not to say that maybe all of the above are right?
Look at it this way, if you may:
Early Mini-comics: Castle Grayskull is full of ancient and mysterious technology. One must control both halves of the PowerSword to gain entry.
This is how it began and Filmation showed a Museum of Weapons within Grayskull!
Filmation: Grayskull draws its immense power from the Abyss.
The Abyss might be where the "Eye of the Universe" is located.
Movie: The Great Eye of the Universe unleashes the ultimate power when Eternia's moon reaches its zenith.
Perhaps Eternia's moon or at least one of them is one of the key to access the power within Grayskull Castle!
MYP: King Grayskull is the source of the power.
Who's not to say that Grayskull is not the only source or perhaps he also gets his power from the "Eye"? The sword did came from "The Abyss" when it was given to Adam, did it not?
Classics: King Grayskull / He-Ro / Overlords of Trolla /The Sword of He!
And here we are! The MOTUC line kinda mixed all and everyone of the above did it not? Perhaps He-Ro and King G are not sources but users of the same power. The Trollan Overlords are what they are. They "Over-See" things in the universe! The Sword of He is like a conduit to access the true power that lies "secretly" within the walls of Grayskull!