Clark Ashton, or E.E."Doc", j.d.?..
CAS, definitely not "Doc". He was a dear man, from all accounts, and a marvelous storyteller, but his prose is... hardly sterling. Also, I didn't even think of him in this context, since we're dealing with fantaisistes, and he was a writer of sf....
I'm going for JRRT as well. I've lost count of the number of times I've read his book, and it never fails to impress me that he can change styles from, say, rustic comedy to "high" English with such felicity and smoothness.
I'd say this goes to what I was saying earlier. To continue to use Tolkien as an example (CAS fits this too, as do Lovecraft and several others): I don't think people realize how versatile Tolkien's prose is, either between the various works or even within the context of LotR. The earlier portions of that tale are written in a light, almost comic vein, slightly bumptious one might almost say -- these are (supposedly) taken from the chapters which Bilbo wrote at least part of, and reflect more of the tone of his own book, The Hobbit; or, There and Back Again; the tone gradually changes as Frodo was supposed to have taken over, becoming more sober, even somber, in tone and word-choice; the "high" tone becomes more and more prevalent as those in close contact with Gondor, Rohan, and their ancient customs and traditions had more input into the work; and finally we return to something resembling (but not identical to) the approach in the earliest portions of the book, as we reach the part which Sam was supposed to have put in order.
Each reflects not only the milieu in which it is set, but also reveals character, in tone, word-choice, and rhythm and sentence structure. These were things Tolkien was intensely aware of, as they formed not only a great interest of his, but also had much to do with his work as a philologist, both in the sense of a literary scholar and a student of historical linguistics ("The study of linguistic change over time in language or in a particular language or language family, sometimes including the reconstruction of unattested forms of earlier stages of a language"). It is exactly the sort of thing he brings to other works, such as the collection of verses published as The Adventures of Tom Bombadil (especially in his introductory note there, where he traces the original documents -- by internal evidence -- to certain personages of the Third Age or earlier) and even, to some degree, with "Farmer Giles of Ham" and "Smith of Wootton Major".
The same is true -- albeit from a different perspective -- with Clark Ashton Smith, who was primarily a poet, and whose tales are an extension of that in many ways. His prose style varies radically from the slightly bookish but nonetheless overall terse and modern "The Return of the Sorcerer" to the arcane usage of terms, structure, and tone in "The Double Shadow" -- or the almost racy, ironic approach of "Something New".
Good prose -- really good prose, not simply serviceable or transparent prose -- not only flows well, it defines character and setting, sets a mood, reflects the internal logic of the world it represents, and consists of very careful choice in phrasing, rhythm, and structure in order to do this.
This is the reason I can think of few modern writers who truly fit the bill of writing "the best prose" -- it may be clear, it may be lucid, but it does not meet the other criteria long recognized for truly great prose writing -- not just "pretty prose", but prose which very carefully serves a myriad of purposes and is deliberately and thoughtfully chosen for that reason.