Cliche answer, but Sword Saint Isshin (Inner Isshin in particular) for me. The only blimp in this perfection of a boss fight is that lightning reversals make his final phase far too easy. The reversal timing is just way too generous in this game for how powerful it is.

For a more unconventional answer, Demon of Hatred. Lots of people say that he doesn't feel like a Sekiro boss and should be in a Dark Souls game instead, but I disagree. Not only does he fit Sekiro's game philosophy, he's one of the game's top bosses IMO.

Glock saint Isshin all the way, the slugfest with him is just satisfying even if he wrecks you in the beginning.

Both Owl fights. The spectacle of fighting against your lifelong mentor was really cool.

Too many to choose one.

Owl in all three versions
Genichiro
Isshin in all 3 versions
Corrupted Monk
Gyoubu

hell I even enjoy slamming the ♥♥♥♥ out of the monkeys with the mortal blade.
Honestly the only "boss" fight I actively dislike is the folding screen monkeys one. Literally every other fight has its unique merits, making it incredibly difficult to choose one. For me the only thing that really separates them is the spectacle/story implications.

Main/Memory Bosses:
Owl Father
Inner Father
Sword Saint Isshin
GYOUBU MASATAKA ONIWAAAAAAAAAAAAA!!!!!!!!!!!!!
True Monk
Great Shinobi Owl
Genichiro...maybe. Like he is mostly a punching bag and I always call him Genshin. If I actually want a fight, then Inner Genichiro.
Maybe Divine Dragon, but that's because I love lightning in this game. That's about it.

Mini Bosses:
Ashina Elite
Long-Arm Centipede
Shadows I guess. I do hate them on charmless/demonbell however.
Shichimen Warrior

Camera.
Really knows how to piss me off.

Originally posted by BananaChumpus:

Camera.
Really knows how to piss me off.

100%.
It's main boss of all games made in Japan.

O'Rin of the Water

Most unique patterns, very graceful and beautiful, not especially difficult once you learn her moveset, but still requires timing and skill.

Sword Saint will forever be my favorite boss of all Fromsoft games, especially with mods like For the sake of Ashina or JiDao. He taught me to love Sekiro

Genichiro!!!

Sword saint hands down the satisfaction of beating that dude is still unmatched.

Dixie Kong's Double Trouble. It's one of the most satisfying fights to do with no lock on, and is the fight that thaught me how to actively use goomba stomps to reposition as opposed to just sprinting.

True Corrupted Monk

Originally posted by Don Lobo:

Dixie Kong's Double Trouble. It's one of the most satisfying fights to do with no lock on, and is the fight that thaught me how to actively use goomba stomps to reposition as opposed to just sprinting.

i bet you didn't even collect all the banana birds.

Demon of hatred is my love-hate boss, he is awesome