Oh man, between Souls III and Bloodborne alone, it would be hard to pick just one. I would have to go with:
-The King of Storms/Endless King (Dark Souls III): One of the most epic boss fights, period!
-Shadows of Yharnam (Bloodborne): I had an incredibly tough time with this fight, but it was so much fun learning how to do it!
-Rom the Vacuous Spider (Bloodborne): I'm so terrified of spiders in real life, but I love playing games with them so....figure that out.
-Deacons of the Deep (Dark Souls III): A pretty easy boss fight, but I loved it!
-Donkey Kong boss fight in New Donk City (Super Mario Odyssey): Definitely easy, and it barely qualifies as a boss fight, but it was just really fun, and cool!
-Nightmare King (Hollow Knight).
-Red Wine Drunk Randy (South Park: the Fractured But Whole)