This is a difficult one. I would say either Volvagia from OoT or Zant from TP. Zant was really easy but the battle scheme was really cool. The second Agahnim fiight from AlttP was pretty fun also. I could keep on going... So, I really don't know. Oh, and the final boss battle against Ganon at the end of OoT was great. Okay, I'll stop now. It's too hard to pick just one.