My favorite boss in DS3 is the Twin Princes. The lore, princes characters and especially OST (which is my favorite in Soulsborne) are just outstanding for me. But the fight itself isn't the best. Nameless King's, Gael's, Demon Prince's and Champion Gundyr's fights are more intensive and stylish.
Nameless king is also top-tier boss. But if you take the closer look, it's obvious that objectively the best boss is Slave Knight Gael. Epic cutscenes, best bossfight arena, Many different attacks, coolest combo in the game (when Gael jumps in the air and shoots with the crossbow), day to night change, lightnings all around the bossfight arena and many others things make this boss more than just a fight.