Ook's battle I found fun, and somewhat unique. Others may have been better, but this one I felt was newer to Zelda. I'm sure others would prove me wrong, but the junping around / bashing poles mechanic seemed new to me. Also, for the music alone. I have no idea why, but it's just catchy. I tend to think of it as Zant's theme since it reprises in his battle, and yet it's hardly his at all.
Overall, I think Darknut was more memorable, but pretty standard fare for Zelda I suppose. Music was great there as well though!