Best? Astetic wise I would say definitely Yhorm, I honestly loved the whole storm ruler sword. It was even better if you got onion bro that far to help.
My personal favorite is the consumed King. His lore. It hits you.
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Some of the bosses in 1 and 2 weren't that good lore-wise. But they really brought that up in Dark Souls 3, where I felt bad for reading lore after killing Yhorm. I realized what a badass he was, and the stuff he's gone through. Good stuff.