I haven't faced all of them yet. Not by a long shot. I've been playing other genres much more often, though recently I've got back into it via sekiro and demons souls remake in anticipation of elden ring. I think my favourite so far might have been... Well let's see.. Phalanx is too easy, the tower knight was pretty great but simple, the asylum demon in dark souls was fun but easy... The drunk dude in sekiro was fun but a lot easier when i realised you could thin out his adds entirely... Lady butterfly was damn difficult and i was psyched to beat her, the chained norseman ogre was pretty great for his awesome animations.. also freaky as a first big boss... the shinobi hunter was easy and i didn't even know how to do mikiri counter when I faced him.. the general by the gate was really easy compared to lady butterfly, so not as special... The fiery bull pretty cool but just a miniboss.. uhh the ashina elite was fucking badass but easy to beat.. genchiro was easier than lady butterfly, maybe because of lady butterfly, easy to dodge his second phase special attack... That's it for me so far, I've got plenty of soulsborn-like bosses that i remember fondly, kena's woodsmith on top of the tower, the wood knight was way harder than other early bosses... the dark jedi boss on kasshyk..
I think for me it's either the white ogre with his amazingly animated body slams and awesome introductory stocks-breaking, or lady butterfly for her iconic "you're dead" line - まだまだ子犬よ!- and challenging variety of attack types, and homing adds that make it all harder.