It's perfectionIshhin from Sekiro is as close as we've gotten to a perfect boss battle IMO:
It's challenging but fair.
It doesn't rely on some gimmick but rather puts your mastery of the combat system and all the skills you've learned through the game to the test.
It looks awesome
It's not some super long and drawn out endurance test. Win or lose it's usually over in less than 10 minutes