Had to choose Sekiro out of those three. I liked Nioh a lot when it came out, but all the loot management you have to do got pretty tedious for me. I didn't really like having to constantly rid myself of dozens of items to clear inventory space. Nioh 2 started out really strong for me, but I found the difficulty spiked late in the game to a pretty ridiculous degree. Maybe that's on me missing a mechanic or something, but it was a case of serious diminishing returns.
Sekiro is a weird game to me. I think as a whole game, it has some serious faults in its level design, it's difficulty curve, and giving you incentive to really explore around you. On the other hand, some of the boss encounters are incredibly memorable and learning the dance, as it were, was rewarding in a way that few games manage. The game could honestly be a boss rush and it would probably be just about as good as the fully fleshed out game.