Ok, I think I've decided on Zaganaga! According to Zeldapedia, Zaganaga is widely considered the hardest boss in the game, but I felt he was one of the easiest. The whole battle is basically one big puzzle. You don't even need the Sand Rod, you can just stand in this spot, use your Swordbeam to damage it when it goes to the platform just ahead, and Merge with the wall to dodge it's beam-attack!