Asriel Dreemurr should EASILY be at the top of the list, simply because of all the power he has with those souls. His story, very similar to Saness, is unknown. This sans is the watcher and protector of all AU's. For pacifist? It would be Asriel in his God of Hyperdeath form.
D Gaster can and will turn off Sans’s abilities if they were in a FIGHT, but he’s not part of the story anymore, Wiki said that when he fell on the void he was scattered across time and space. 'Undertale' gives you a setup and then truly makes you feel like you are the most powerful driving force in Toby Fox’s scripted scenarios of good and evil within you, the player, happening in the underground.Despite being a monster, Mettaton idolized the humans, and believed that if he had a body, he could
The player meets various monsters during the journey back to the surface.
There are a lot of similar character archetypes between the two games. Who is the most powerful character in undertale