This is a FromSoftware game were talking about here, and after the countless examples in the Soulsborne titles, people had high expectations for Sekiro's boss battles. And as this list here shows, they were fully met if not shattered. Unmarked spoilers ahead.
- Gyoubu Masataka Oniwa - an armored Samurai on horseback, wielding a massive yari that he can sling around on a rope with an incredibly memorable introduction? Hell yeah.
- Lady Butterfly is one of the most badass bosses in the game, constantly darting above you on invisible wires, throwing her kunai at you, then launching at you with kicks that would make Chun-Li proud - and that's just her first phase! Combined with the gorgeous burning temple arena, she is one of the most memorable opponents you get to face. If you enter the Hirata Estates memory as soon as it becomes available, she may even be the first proper boss you fight, making the encounter even more memorable.
- Genichiro Ashina. The first time you fight him, it's a mostly an unwinnable boss fight. The fight after that, though? Genichiro is every samurai general taken Up to Eleven, with a fast-firing bow and arrow combo to boot, and while you have gotten better, it is still an extremely challenging fight. After you beat the first two phases, he strips down his armor, imbues his blade with lightning and becomes Genichiro, Way of Tomoe - providing an even more fast-paced battle with the addition of powerful lightning attacks.
- The Guardian Ape, a giant ape with a huge nodachi stabbed into his neck and an immortality centipede infesting him, is also up there - mostly for being a memorable entertaining fight as well as a shocking Player Punch surprise when he finally goes down... only to suddenly get back up, picking up his head and attacking you with the sword you just used to decapitate him. Apparently devs liked the fight too, since you get to fight it again later, except by then it has another giant ape join the fight.
- The resident Mirror Boss, Great Shinobi Owl, provides one of the most epic examples FromSoftware has ever put in their game; an emotional duel to the death against someone Wolf loves, all the whilst Owl unleashes a full arsenal of shinobi tools, fast swordplay, and insane acrobatics to stomp you into the ground. It is somehow topped by the optional fight against Owl in his prime in the Hirata Estate. His attacks are even faster, he uses even more shinobi skills and tricks, and in the second phase he summons a spirit of the owl for teleportation and fire attacks. When you beat him, it truly proves that you have surpassed your father.
- The Divine Dragon. While an easy fight overall, it has beautiful music, it is visually gorgeous and its mechanics make it akin to a theatre spectacle performed by two world-class acrobats: the Dragon slices with a massive sword, creating barriers and blades out of air, while the Wolf swings from trees, reflecting lightning bolts at his opponent.
- No From game would be complete without a brutal Bonus Boss, and The Demon of Hatred is as threatening as it looks. What remains of the hate-consumed Sculptor is a relentless beast that hits like a truck, has a number of deadly combos and transitions quickly into different attacks. You'll require three death blows to put it to rest and with each death blow it adds new lethal attacks to its already powerful repetoire, including giant waves of fire, homing fireballs and even a ring of fire to trap Sekiro with it in a small enclosure. When you first arrive at the area where you once fought Gyoubu you are just in time to see it wipe the floor with the few Interior Ministry Red Guard soldiers left standing, with the arena littered with their corpses. Good luck.
- Sword Saint Isshin. The very final boss if you side with Kuro and one of the most ludicrously difficult enemies From has ever made (easily rivaling bosses like the Orphan of Kos), it should be no surprise that he provides one of the most memorable final battles in the history of From Software games. It starts off with a final rematch against Genichiro, who duels you with the black Mortal Blade on the same field where he cut your arm off at the beginning of the game. This battle is quite good on its own, but it's only an appetizer for the real fight. Once you break Genichiro's posture, he realizes he can never defeat you, turns the Black Mortal Blade on himself, and summons the deceased Isshin back from the underworld in his prime, slumping to the ground as the old warlord literally pulls himself out of the wound in his grandson's neck. What comes after is a Nintendo Hard duel between you and a legendary swordmaster with a katana, giant spear, pistol and lightning powers on a beautiful, moonlit field of silvergrass, while Ashina Castle burns in the background and a storm rages around you, imbuing Isshin with the power of lightning.
- On the other hand, siding with Owl provides another excellent boss fight, this time against Emma and the elderly version of Isshin. Emma is a Lady of War to rival Lady Maria, easily able to keep up with Wolf in swordsmanship and boasting attacks that can one-shot you, while Isshin is an Old Master who attacks with such speed and precision that some of his sword strokes are literally invisible to the eye, later augmenting his attacks with fire as well. The battle provides an excellent denouement for the Bad Ending of the game, as you are forced by the Iron Code into a tragic and unnecessary duel against your former allies, who will put their lives on the line to stop you from becoming a Shura.