Although Bruce is great I have to go with Dio.
Ronnie James Dio's musical career is just amazing. Spanning nearly 50 years! From "oldies style" rock n roll with Ronnie and the Redcaps and Ronnie and the Prophets (1957-1967), to blues rock with Electric Elves (1967-1969), The Elves (1969-1970), Elf (1970-1975), and hard rock with Rainbow (1975-1978), and pounding, dark Heavy Metal with Black Sabbath (1980-1982 and 1991-1992), and his own self titled band Dio (1982-present). Plus a bunch of other side projects and special events.
His voice is truly one of a kind and never to be imitated.
One must really admire such a musician with so much talent and experience.