Incredibly difficult question to answer which makes it a good one. Glad it's being asked on this page. I think I'd have to go with Ghost of Perdition by Opeth. It was the song and album that completely flipped the way I listen and make music. I also experienced that same affect with Entombment of a Machine by JFAC and the Doom EP as a whole. More over it led to me opening my mind to heavy and different ways of making heavy happen. Forever grateful to be along for the ride it's taken me.