We’re not putting the other Alma this high up on the list because of her inherent humanity or bloodline relationship to Rachel. She earned this place because she completely, utterly dominated us when we first encountered her. Never before had a boss flung us around as if we were a used tampon. Once we figured out that we needed to constantly jump/roll/jump/roll to dodge her attacks, things got easier. Or rather, the battle became fair.
She embodied everything that was Ninja Gaiden: hard as hell at first, but almost never cheap. Once you figure out how to take out the enemy, the game was still challenging, but rewarding.