Who is your strongest character?

[Amaterasu]

It would come down to a battle between Amaterasu and Undyne. Undyne has the advantage of being a strong fighter at close, medium and to some extent high range, whereas Ammy is best placed in a close range but not melee. On the other hand, Amaterasu wields T2 Supers and has a more tactical approach to things than Undyne, which the fish-warrior lacks. They are both experienced warriors too, so that won't be the breaking point.

Once Amms gets her mega-purchase though, she'll pummel Undyne into the ground.