I don't think "greatest" has to be "entirely subjective," though that may be how some responders take it. I do think that a lot of fuss could be avoided if people who propose topics such as this one would provide criteria at the outset to help to guide the discussion. In this case, I think criteria can be suggested according to which the discussion would be more than just an exhibition of fans naming their faves.
"Greatest" could be understood as (in this context -- the discussion of an sf writer) meaning: the writer whose work intriguingly explores a wide range of what have come to be established sf concerns, earning recognition from a wide range of readers (numerous countries, and differing generations) for that, and deploys the literary skills that multiple generations have found, and continue to find, to be effective. I should hardly need to say, after offering that definition, that my nominee is H. G. Wells.
Wells isn't the first sf writer, and perhaps some would argue that any one of his stories is surpassed by some greater work,* but he was the first writer who really made sf his own, writing standard works that remain fresh and compelling not just to me, but to you, to a Czechoslovakian university student in the 1960s, to a Canadian veterinarian on the prairie in the 1920s (these are hypothetical, but believable, readers), etc.
*Thus someone might argue that (___) is a greater invasion-from-the-planets story than The War of the Worlds, (____) is a greater time travel story than The Time Machine, (____) is a greater tale of a startling invention or discovery that demonstrated things about the human moral condition, (____) is a greater horror story of tampering with biology than The Island of Dr. Moreau, "_____" is a great tale of the glimpse, through a mysterious alien artefact, of a weird world than "The Crystal Egg," (_____) is a greater evocation of space travel to a new world and of an alien culture than The First Men in the Moon -- and so on. But these were early and fresh ventures in these categories and they are very well written; evidently they translate well, by the way. Now what other author has an achievement like this?