Board Thread:General Discussion/@comment-26347196-20160410032827/@comment-27701762-20160411222429

I'm going with Sans, as ultimately both battles rely on very similar skillsets (memorization plus reaction), and the key differentiation is that Sans's battle is simply longer, meaning that there are more patterns to keep in mind. Undyne can be defeated more or less quickly depending on skill with the battle menu and choice of weapon, while Sans always requires the same number of attacks.

Undyne's worst attacks are probably the slow-moving arrows (since they have no specific order and require good forethought) and the spear circles. The rest of the attacks just require that you (even vaguely) remember the attacks. Even the yellow arrows are simple once you remember the basic pattern.

Sans's worst attacks are the slam and the blasters, and of course the penultimate attack (I also greatly dislike the short-hop bones in the first half of the fight). Pretty much every attack has some little trick to it that alleviates a lot of the pressure, as long as you know the trick well enough.

Having beaten both more times than I've been keeping track of, I reiterate my vote for Sans.