Megalovania and Battle Against a True Hero are my two favorite songs in the game, but that’s 2 songs so I’m giving it to Pacifist (the highlights are Core, Bonetrousle, Undertale, and Hopes and Dreams)
There’s a mod that gives Skeletron the eye and replaces boss one with Megalovania and even makes Eater of Worlds into a Gaster Blaster (sadly that’ll have to be removed)