In my opinion (and it’s just that), I’ve always thought that Tom Jones is bettered by thousands of black singers, but is famous because he’s white and almost as good. A bit like Elvis, Eminem, Jerry Lee Lewis – maybe even Joe Cocker.
He’s been clever (or at least employed clever people around him) to sustain his career.
So I’d put Bowie/Freddie in above him because I think they had an amazingly authentic sound.
But with these things, we’re always going to think of the most popular/famous and then think who was a great singer.
I haven’t seen Van Morrison mentioned, but he might fall into my first category also 🙂