You'll have to clarify your question. Do you mean "author" in the literary sense (e.g. Fitzgerald or Atwood) or in the academic sense (e.g. an economist like Galbraith?)?
Greatest is a subjective term - what is deemed great by you is probably complete shite to me. Simply winning a Nodel Prize, as a matter of fact, is no measure of greatness. It's a measure of meeting the criteria set forth by the prize's governing bodies. And that says nothing of the fields of study for which Alfred Nobel decided not to support with his endowment: mathematics, history, philosophy, etc.
The greatest author of the last century, in my opinion, to never recieve the King of Sweden's honour is the great French historian of the Annales school Fernand Braudel.