Be it in terms of gameplay, story, music or anything else that makes them be the best-designed in your opinion.
I think Isshin from Sekiro is definitely up there. It's a fight that keeps you on your feet, is super challenging at first, but allows for good improvement and still a very fun and cinematic fight. It's a very good ending for an amazing game.
Ludwig in Bloodborne's Old Hunters DLC is amazing too. His transition into phase 2 is one of the most cinematic I've ever seen in any game. The soundtrack is amazing and technically very engaging, his lore is one of the most tragic in the entire game. Speechless.
Who's your favourite?
Dancer of the Boreal Valley is one of my favourite bosses ever. Her introduction cutscene and overall design is just peak. Creepy and menacing, the veil and the slouchy walk. Badass dual swords. The spin attack.
On my first play through I went to the Dancer instead of Vordt and proceeded to face plant like 50x into the boss before beating it.
Then turned around and one shot Vordt
I love that the spin has a timing to it, so that you can't just spam roll.