Best: Kalameet, from Dark Souls: Artorias of the Abyss. Genuinely a fair but very difficult fight, and it never felt cheap for a minute. I also love that you can cheat and get the giant to hobble the poor fella if you can't beat him in his full strength mode.
I was always under the impression this was the normal was to go about things, and being able to take him down yourself was for specific builds? Like, certain builds could NEVER be able to do that. Agreed though, Kalameet was one of my absolute favorites.
Best: Ludwig: The design, the music, the cutscene to and the second stage? I was screaming in my seat it was all so amazing. Also very difficult, but I only played it on NG+, (stupid move), so I can't give a fair assessment on that.
Soul of Cinder for the same reasons OP said.
Artorias because why wouldn't fighting Guts be amazing?
Lady Maria is also a personal favorite of mine for essentially feeling like a DMC boss.
Worst: Smelter Demon: Maybe it was the patch version I played but his hit boxes were garbage and he was just genuinely un fun to fight. Humanoid boss fight number umpteenth, this time with irritating fire AOE damage.
Micholash the chase fight, enough said there.
Bed of Chaos for obvious reasons again.
I don't even count the Ancient Dragon just because it felt like a bad extra boss fight for maxed out characters.
Some many bosses I am leaving out on both sides though. And I've never even touched Demon's Souls.