I was lucky to grow up listening to an awesome rock station. They played lots of nu-metal around the time I got into rock, but they also played a shit ton of classic rock, and had a program called Midnight Metal which introduced me to real metal. I had heard Ozzy and loved all the early Sabbath stuff, but hearing Dio blew me away. A melodic voice that could turn menacing at the drop of a dime, all while being smooth as hell.
If I made a list of my favorite rock/metal sings, Robert Plant, Dio, and Chris Cornell would top it.