So those that have killed most (if not all) bosses, which one was the hardest?
Mine was undoubtedly:
Storm/Nameless King. I finally killed him after countless tries. Not possible until you fucking perfect the dodges on his ultrafast shit. I got killed in second phase so many times that I could probably do the first/dragon phase blinded. I think it's safe to say that for me it was the hardest boss in Souls series.
I think a problem with the boss design in mid-lategame is that some of their movesets can be facetanked while spamming to deal heavy damage to them while losing almost nothing as estus is extremely fast and plenty avaiable like
pontiff when he starts summoning his shade/copy, aldrich when he starts charging his fadeaway attack or the younger prince when he's doing that wrath of god attack reviving the other prince.
However that one is really well designed and punishing if you fuck up, took me so many tries I lost count, I went to his area first when i was level 60-something and got killed 10 times before calling it a day and switching to another area, killed him today at sl94 but it still took me way too many tries even using straight sword R1s, yoloed it at the end after in a try I got the stagger-riposte on him somehow mid phase 2.
He definitly takes the cake for me not only as hardest boss of the game but also as most fun as the atmosphere when you fight him is just awesome.