Between Asylum Demon, Tower Demon, Witches of Hemwick and Genichiro, he probably takes the cake for me.
At first, he's a huge wall and pretty much impossible to beat for new players. Once you get the hang of the combat, it just clicks (literal shills running down my spine) and you end up appreciating the fight a lot.
What's your pick?
Of course, i was refering to lord of blood. It felt like if the entire game was just meant to prepare me to fail that fight. I really had to learn how to dance to his liking. Mohg's second phase is so chaotic and you have so little time to take decisions that i really dont know how i beat him in the first place.
It was really hard but beating him was so rewarding that it made me understand what a good boss is soppose to be.
It's a strange thing to say, but Mohg was plain hard and i wouldn't call that fight unfair in anyway. I wish there were more bosses like him.
And, of course... the music.