Hardest boss is definetly Majora's Mask itself in my opinion, considering you're fighting it without the Feirce Deity Mask. Gyorg wasn't really hard, maybe even one of the easier ones. I do remember when I was young and you had to jump in the water to hit it as Zora Link. When I hit it with my electric barrier shield I was like ''sh*t, sh*t, sh*t, gotta get out of the water'', because then he would come after you and eat you . Really gave me an adrenaline rush when I was younger.
Best one still is Goht though. Most original boss in the game. Twinmold was the worst. Gyorg itself was pretty decent.