What are some singers who have or had great careers but would have gotten put down by Simon Cowell when he was a judge on "American Idol"?

I often hear entertainers who have good to great careers but do not have spectacular voices on the radio or from other sources. In fact, if they had to sing a cappella with no music behind them, they would sound lousy. What entertainers would you place in that category? I know some people would name Janis Joplin who had a great career but to some not a great voice?