Board Thread:General Discussion v2.0/@comment-26347196-20151109173852/@comment-27354197-20160111031718

In my opinion, it has to be Sans.

He may not be going all out on you, as seen in his dialogue path, but he is the only final boss where you cannot respawn and/or save during battle. With Omega Flowey, the game saves for you and you start off there. And when you get close enough to the end, the human souls begin to throw you healing items so you don't die as often. During Asriel's battle, Frisk's soul recreates itself through sheer willpower and determination, and you continue the battle.

But during Sans' fight, you cannot save or respawn anytime, and each time you die you have to start again.