It shouldn't matter what the original song genre(s) were, just as long as it was done in a metal music style for the cover.
Some current favorites: Judas - Rage of Light (Lady Gaga cover) https://www.youtube.com/watch?v=Z7cvquM3zNc&t=287
The entire album "Epica vs Attack on Titan Songs" by Epica
Powerwolf's multiple bonus albums where they have other bands cover them or collab. Interludium, Communio Lupatem. This gave us the awesome Battle Beast cover of Resurrection by Erection, where you can tell they were having a blast.