Man, this is a tricky one! I think if I had to choose just one to be 'the greatest', it would have to be Paul Auster. I think his books are definitely literary, and he must have an incredible brain to have come up with some of the concepts he does.
Although - if I can be cheeky and choose two - it would be Shakespeare. Not only did he write those incrtedible plays, but he invented new words like 'eyeball' when the words of the time just wouldn't do - and we still use them today! Now that's got to be real evidence of a fantastic genius at work.