For what he does Dio is tops. You really would be hard pressed to find another vocalist with the same range and power. His range is around 5 or 6 octave, and the man could easily be working in opera if that was his chosen path.
Thom Yorke has laid down some beautiful vocals, but his strength is how he uses his limited ability to best suit the song.
Henry Rollins was one of my heros growing up, (and a super cool guy, I\'ve actually had the chance to hang out with him) but he\'s a pretty one dimensional vocalist.
MJK is unique and impressive. A favorite of mine, but no Dio either.
But seriously Paul, what do you know about vocals? You listen to Rush.