I'll have to say the Fierce Deity Mask, and for three reasons. You can wear it to assume Fierce Deity form. And while wearing this mask, you look very much like your normal form, but older and a lot stronger. Your sword is now two handed and does a significant amount more damage. Also, while Z targeting, you can shoot beams at your enemies. Plus in the Japanese Version, people call it the Oni Link.
The Power of Three
It's just The Power of Three