Sekiro definitely felt like it demanded the most from me. It also had a much longer learning curve than the other games. I felt like a bumbling idiot for like a whole third of the game before it clicked.
I agree about the builds and I originally thought the playstyle stuff was very strict too, but from watching videos of other players, I'm not so sure anymore. I see people taking on attacks very differently from how I did them. Running away from certain attacks when I tried to punish them, or dodging and punishing attacks that I parried. There's more variety to the combat than I originally thought.Dark Souls 3 gets incredibly cheap and stupid near the end (especially when you count the DLC). Sekiro is the "hardest" in the classic sense of rote memorization, though. Sekiro is either you can do it or you can't, there is no room for alternate strategies or builds.