Ludwig the Accursed/Holy Blade from the Bloodborne DLC. It gets pretty much everything right.
Very challenging but fair for the most part and it's pretty fun when the "rampaging beast" archetype can be hit or miss in that department with a second phase that switches to a hybrid beast-duel boss and is introduced with a fantastic cutscene as the music switches to this creaky, haunted tune to this beautifully heroic melody as you see a man trapped in a beast's body regain his conscience and his dignity and now wants to fight you as a man. It's a genuine thing of beauty.
There's a lot of good ones, many already mentioned like Armstrong, Gael, Shibusawa (though Awano is personal favorite Y0 boss) and even a shitload of other Bloodborne bosses like Gehrman, The One Reborn or the Cleric Beast but my vote's on Ludwig.
A honorable mention to Shido in Persona 5; a very drawn out fight but it's pretty fun with a kickass theme song and a lot of cool imagery. And I wish Adam in Automata got more development both as a character and a boss, the potential's there but never feels truly realized.