Hard Bosses

I wouldn't really class any of the bosses in any Zelda game as "hard" (besides AoL - Thunderbird was challenging).

However, in terms of the most annoying, my votes go to:-

OoT - Phantom Ganon.

MM - Gyorg (The mask's first phase was also particularly annoying when MP runs out, and instead of bringing potions, you have to use Zora Blades if you don't have Fierce Deity's Mask ¬¬).

Hmmm....Morpha Really gives me trouble, I just can't seem to avoid his little aquatic appendage...Just couln't get that nucleus.
Ganondorf was pretty challenging, because I have bad reflexes, so it was hard to win the bounce back part, then when I did when I didn't get him with the arrow fast enough, or else I missed.
Goht was hard the first time, I couldn't avoid his little projectiles.
Gyorg, because you have to get into the water to effectively attack him, and you can't stay above or below water for to long or he took three hearts from you.
I've never fought Twinmold, so IDK after that.

The hardest boss I would have to say is ganondorf on OOT because the rest of the bosses were cake. Probably other than that is twin rova.

i would say Phantom Ganon it takes a while to get the timing down for me when he throws the stupid energy blasts at you!