Right. You guys have all been waiting for me to give the final and definitive verdict. But I have none to give. Sorry!
Who is Greater? In what respect exactly? Writing, style, literary aspect, genre-changing or -defining aspects, rate of influencing of those that came after?
I love SFF and am an avid reader, but I doubt I could pinpoint any writer and say he, or she, or they, was The Greatest. I like one writer more than the other, but that is not saying much. Any preference might also have its origins in the order in which I read their work.
When Dune was first published in the Netherlands (somewhere around 1973/1974) the books lay in large stacks prominently on the presentation tables and counters. I had never seen that before with any SF or Fantasy novel. It was a door-stopper and had an intriguing cover. Everything shouted 'this is something special.' and 'buy me!' So I did and never regretted it. It was SF unlike anything I had ever read. To me, while reading it, it felt as if SF has matured. It was complex, layered and deep. Dune still sits prominently among my favorites.
Foundation somehow escaped my attention in my younger days and I didn't read it until a few years ago. I read book 1 and 2 (or only half, I can't remember) and never could be bribed, bludgeoned or blackmailed to read the remainder of the series. Too dated to me taste. That doesn't mean it cannot have been as important to the genre as Dune in my opinion was. I'm not sure exactly where I shelved his books.
I have however read a lot of Asimov, in those now somewhat misty days of my younger self. I liked them, but never thrilled me.
From Herbert I read (of course next to Dune. Dune Messiah and Children of Dune) The Dragon in the Sea and The Santaroga Barrier. Neither convinced me to continue on that path.
All in all I think Asimov was the Greatest when talking about the importance to the world of SF. He made it accessible and popular to the general public. Next to Arthur C. Clarke.
But I think the one novel that had the most impact (at least within the SF-world) is Dune.
To be honest though, I dislike terms like The Greatest. Let's just say Among The Truly Great.