Why not name some? I beat the standard game(no dlc) and the only memorable fight was the lightning dog.
All of them.
Lightning Dog, Smoking Frog, Laser Slime, Huge Skeleton, Seven Heads Snake (I like how you called it, so take those, lol).
But specially the human bosses. The sword fighting on Nioh is by far the best ever. Fighting other chars was amazing.
Even more in the two vs one fights.
People say Dark Souls Ornstein and Smough was amazing... lol.
Yuki Ona and Oda Nobunaga
Honda Tadakatsu and Tachibana Muneshige
Those are the real deal, nothing compares.
Nioh is by far the game with the best boss battles, gameplay wise, not only of this gen but of all time.