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Mine was definitely Pontiff Sulyvahn, took me a while to beat him. Probably because I was using a +4 long sword but whatever :p. I haven't beaten the game yet so I obviously don't know all bosses yet, there's probably a harder boss than Sulyvahn :p
Edit: Nope Nameless King is harder, holy shit it feels like it's impossible
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Abyss Watchers. He's the only lord of cinder I've killed so far. I've killed 4 other bosses besides them.