I don't know if I'd go that far, but according to the Facebook group (which I recently took over as administrator) he is.
The original creator of this group titled it so. Now, there's no doubt Winston influenced hundreds if not thousands of pianists to play in the New Age style and I'm certainly one of them.
But to call someone the greatest is really a subjective thing. How do you define great? By how well someone plays technically? I doubt it. Is greatness defined by how well the music effects someone's emotions? Then Winston certainly ranks up there among the best.
That's why I never attribute terms like this to artists of any ilk. Doing so sets up a comparison trap that really serves no other purpose than to inflate egos who need to have a register as to how well someone can play.
Me? I prefer to keep silent on such matters and let the music speak for itself.