Hard to choose a favorite (Stupendium did a fantastic Spamton song, Ad Infinitium), but the Undertale album Determination by RichaadEB and Amie Waters definitely ranks up there.

Rich’s red side features explosive and intense metal covers of the iconic battle music. Ace’s blue side features chilling atmospheric and electronic renditions of each of the area themes, as well as a few other tracks. Together the two sides make up Determination

https://determination-ut.bandcamp.com/album/determination