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?
Visually cool design or in terms of gameplay? I didn't like the gameplay too much personally.
I think there's definitely cool bosses but some are a little too "filler-y". Like those watchdogs, for example, or these crystal dudes in the tunnels
Both visually and gameplay. I think most bosses are terrible at telegraphing attacks in ER, but this one does a pretty good job. You need to pay attention to both the head and body in different ways that I like.