I agree with Bradbury. Then there's John Steakley (okay, not the most prolific writer), or one of my favorites, C. J. Cherryh (very prolific but not everyone's cup of tea). I personally don't think any single person can or should claim that crown. Everyone is going to pick authors whose work appeals to them personally and each author has their merits, each their own contributions and so to name one, to me, lessens everyone else's contribution to the genre.