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?
Gale has to be it for me, he is aggressive and has a lot of flashy attacks but never feels unfair.
I also think adding the cape to make dodge windows tighter was a brilliant move.
Ludwig is a close second but the attack where he drops from the ceiling knocks him down to second place. I find that it is safer to let him hit you than risk the double damage you take while dodging which is just poor design.