Haven't fought too many final Zelda bosses but as of rn, I'd have to say ALttP Ganon. The first few phases (where he teleports around the room and shoots fire) can deplete your health really fast if you're not careful, and hitting him is tough. Once he turns the lights out, it gets even harder because you have to line up your shot with the Fire Rod, light both torches quickly, run over and hit him, then line up your arrow shot. Then you have to repeat that like 4 times. I fell off the stage so many times and had to restart the battle a lot.