I gave this some thought and can't think of any better metal vocalist that I've actually seen live.
Rainbow in the Dark is one of the best heavy metal songs of all time. Saw him in several interviews not too long ago. Guy was a class act. Nothing like his persona on stage. He had an amazing talent and was able to keep his head on right. Rest in Peace.
Very few can match...Geoff Tate, Phil Anselmo, Bruce Dickenson.
Joe Lynn Turner (Dio's eventual replacement in Rainbow) was stronger, but not by that much. Rainbow was awesome and they're one of the few groups I ever bothered to see in person. I wish the group had worked out better.
RIP Ronnie.
Halford for pure vocal power and range, Dio may have an edge in showmanship, though.