There really is no greatest author ever, IMHO. It is all based on your tastes.
In my opinion Stephen King and John Grisham are the greatest authors ever. People pick Hemmingway and Ayn Rand but most of their stuff is hard to read and you have to think about what they are saying and that just takes the flow out of reading.
However without a doubt, the all time greatest author is God. After all He did write the Holy Bible