I'd say the Goron and Zora masks are tied at the top. Fierce Deity is more powerful but you're limited to using it in boss rooms (unless you do the glitch to do it outside of them) after you've already beaten all the bosses except the final one so it's not that helpful at all.
For non-transformation masks I'll go with the last four I hold onto in the moon dungeons: the blast mask, the bunny hood, the mask of truth, and the stone mask in that order (meaning the blast mask is the last mask I'd give up).