Personally I've greatly enjoyed pretty much all of Demon's Souls bosses. I liked the mix between your classic FS bosses like Flamelurker, Maneaters, etc, and puzzle-gimmicky one's. If you're playing for the first time and trying to find those solutions by yourself, it's a joy.
I guess I would be in minority, but I think that overall FS has stepped down in terms of boss designs since Demon's Souls. Their art designs became better, sure, but the gameplay is a rough copy-pasta now. Look, Bloodborne is one of my favorite videogames of all time, I've finished it literally 10 times already, but every single boss is fucking same in terms of gameplay design and thus your strategy is the same. It's just some thing that hits fast and hard, their appearance is different, yes, but your strategy is always the same – physical practice. Learn their patterns and try to be good at dodge timings. Dodge, dodge, hit, dodge, hit, etc.
I would actually prefer BB or DS3 to have more of those Demon's gimmick bosses. In terms of gameplay, finding how to beat Fools' Idol, Dragon God, Storm King, Tower Knight was way more fun and interesting than fighting Amelia, Amygdala, Gehrman, Lotric, Sister Friede, etc, because the formers felt unique and I had to actually think about how to beat them, the latters (although being crazy visually-wise) demand 0 thinking from me, only to master my muscle memory. And to me it's a step down for sure. Yep, I won't forget how BB's bosses looked or how tough they were, but other than "I've spend 3 hours trying to kill it" there's nothing to remember in terms of gameplay for me.
And again, I don't need all of the bosses to be puzzles, but they're kind of needed just to fresh up overall experience. I honestly think that we don't need all of 20-30 bossfights in the game to be different only in terms of visuals. But yet again, I'm not playing FS games for the challenge amd hardcore, I play them mostly for their lore and environment story-telling methods, so I guess it's a "me-problem" kind of thing.