I'd give the Wind boss an A, the Fire boss a B, and the Water and Lightning bosses a C. I think the Wind boss had a creative fight with multiple ways to defeat it. You can beat it without even using any weapon at all by diving through it which is incredibly satisfying. The music for this fight is also excellent. The Fire boss felt like a pretty typical Zelda boss, and Yunobo feels the most like a traditional Zelda item of the sage abilities. He's basically like a Megaton Hammer with range and cooldown. Water and Lightning I think both suffered from the cooldown of the sages dragging out the fight and making the fights much slower than they could have been. In retrospect I learned that the Water boss could have been more fun if you just use an opal attached to a weapon rather than waiting for Sidon's cooldown to end. Also the Lightning boss has a little more
An idea I had was that the game could have been better if the sages were just replaced with traditional dungeon items since that's essentially how they function. Yunobo would have been a ranged version of the Megaton Hammer, Sidon would have been a water based version of Nayru's Love which also could have been based on a magic meter, Tulin would have been a wind burst item, and Riju would have been like the Ether Medallion from A Link to the Past. Mineru is trickier since the construct itself makes sense to follow you around so maybe that would be unchanged. They all could have been magic based except Mineru so you had a magic meter, except I think Yunobo I would have only made the ranged attack magic based so that you could use his item as a boulder breaker that never breaks which I would have loved.