Someone didn't pay attention to the story
For me, my ranking for DS1 bosses probably goes as follows:
Artorias > O&S > Seath > Gwyn > Sif > Nito > Quelaag > Pinwheel > BoC
The ones I haven't included I either haven't fought in any playthrough or just didn't find them interesting enough to include
Artorias is probably the most difficult and perhaps the least formulaic boss I've fought in any souls game. Formulaic isn't necessarily a bad thing, it's extremely satisfying to be able to overcome a difficult boss with a 100% success rate, but it does get boring after a while. Artorias is really good at keeping me on my toes and the only time I've ever managed to cheese him was by speccing for full Havel's with fast rolls. Same goes for O&S, though O&S I managed to cheese with Wrath of the Gods.
Seath, Gwyn, Sif, and Pinwheel are my favorite bosses lore-wise, but only the first 3 really pose any sort of challenge and pinwheel goes down incredibly fast even on a relatively low-leveled character.
Quelaag is easy but bullshit in her own right, BoC is just bullshit.
As for Dark Souls 3, I'm gonna have to say
Soul of Cinder > Champion Gundyr > Dancer > Abyss Watchers > Oceiros > Aldrich > Pontiff Sulyvahn > Lothric and Lorian
This is a bit of a different case of ranking. I love the first 3 just about equally, beat Abyss Watchers consistently enough for me to be sick of them, have a love-hate relationship with Oceiros because his whole shtick is that he's tanky AND deals damage, fucking hate Aldrich because his actual fight is boring and his character is simply terrible, and Sulyvahn and L&L are just way too formulaic to justify their difficulty.
Really, I love Dark Souls 3, but I feel like it tries way too hard to be Bloodborne.