My favorite is definitely Kurt, his voice had this quality that you got in singers like Dave Mustaine, Janis Joplin, and Ozzy's early work, where he just went all out, not caring how high his voice was, or if the melody was lost, or if he cracked on higher notes. There was no posturing or front put up. I'd put Layne in second, followed by Cornell, who's still one of my all time favorites, but all four are great.