I wouldn't rank Farmer among the best, but I don't perceive that you are suggesting that he should be. His Riverworld and World of Tiers series are entertaining and I agree that they are pulpy
but Farmer was known for updating the classic sci-fi pulp sub-genre with more sophisticate sexual themes and and incorporating the mundane, unromantic, but clearly essential requirements of everyday life, like having his heroic characters take the time to find a place to squat and defecate. It's difficult to read Farmer today and appreciate that his work in the 50's and 60's was considered controversial and fairly cutting edge. To a large degree this was due his willingness to address religious themes in his work (as well as the sex stuff).
Personally, I very much enjoyed the work of the prolific Poul Anderson, especially his forays into fantasy which, generally, reflected his Scandinavian heritage and included retellings of Norse legends and sagas (e.g. "Hrolf Kraki's Saga") My favorite among his fantasy novels is "The Broken Sword."
I consider him an excellent, imaginative story teller, who is particularly successful in his treatment of alien cultures (e.g. "Fire Time").
As an aside, Anderson's daughter, Astrid is married to sci-fi writer Greg Bear. They have two children, one of whom, Erik, is also a sci-fi/fantasy writer who has co-authored, along with his father Greg, Neal Stephenson, Mark Teppo and others, The Mongoliad a historical fiction trilogy that has at it's center piece the Mongol Empire in the years following the death of the Khan of Khans, Genghis. The origins and means of collaboration on the three books is a very interesting tale all to itself (See https://en.wikipedia.org/wiki/The_Mongoliad) I am about half way through the third book of the trilogy and can highly recommend it. Anything written by Stephenson and Bear is bound to be very good and these are.
Unless someone completely trashs an excellent writer or effusively praises a hack, I don't argue with taste in authors, and yours is a fairly ambivalent assessment of his writing. He did, however, win 7 Hugo and 3 Nebula awards and both are considered quite prestigious in the world of sci-fi. He also won the Damon Knight Memorial Grand Master Award from the Science Fiction & Fantasy Writer of America. This too is a prestigious award with past winners including Heinlein, Clifford Simak, Clark, Asimov, Alfred Bester, A.E. van Vogt (Whose work, ironically, was detested by Knight) Andre Norton, Ursula LeGuin, Harlan Ellison, Pohl, Bradbury, Sam Delaney, and Larry Niven (In all the award was presented 31 times in 40 years). However, I can hardly credit it as an infallible identifier of excellence in Sci-Fi and Fantasy, as I am not particularly a fan of all of it's winners and there are numerous deserving authors who have been overlooked.
I try to read as many winners of the Hugo & Nebula awards as I can manage (or care to), but while they are widely considered the genre's highest honors, they too can't be considered clear cut marks of excellence. They were patterned after the Academy Awards and votes are cast for nominated works, by members of The World Science Fiction Society (In recent years winning works received around 5.5 thousand votes) Over the years it seems that certain authors enjoyed periods of intense popularity such that they repeatedly were nominated for or won the awards. Some critics of the Hugo maintain it is essentially a popularity contest rather than an assessment of literary merit and value. To some extent, I suspect this is true, but, for the most part, the nominated and winning works are worthy of serious consideration. The best, IMO, though don't always win.
In 1975 Ursula Le Guin's "The Dispossessed" beat out Anderson's "Fire Time," Phillip K Dick's "Flow My Tears, the Policeman Said," and Larry Niven & Jerry Pournelle's "The Mote in God's Eye," for Best Novel
In 1991, Lois McAllister Bujold's "The Vor Game," beat out David Brin's prophetic "Earth," Dan Simmons' "The Fall of Hyperion" and Greg Bear's "Queen of Angels," for the same prize. Bujold is a good writer and "The Vor Game" was a very enjoyable read, but "The Fall of Hyperion" was better and "Earth" and "Queen of Angels" were both, very clearly, superior. Bujold went on to win the award again in 1992, 1995 and 2004 and was nominated in 1997, 2000, 2002, 2011, and 2013. Six of the eight works were the latest installments in the seemingly never-ending series, "The Vorkosigan Saga," which are the tales of a space-faring, deformed dwarf who also happens to be an aristocrat serving as an officer in a galactic empire's military. The other two are Bujold's first entries into the fantasy genre, a retelling of the lives of Queen Isabella and King Ferdinand of Spain, that appears poised to be the beginnings of another multi-volume series that will enrich Bujold.
The Nebula is awarded by the Science Fiction and Fantasy Writers of America, and as one might expect many of the authors and works nominated for Hugos were also nominated for Nebulas. As with the Hugos there have been, IMO, notable "misses" with the Nebulas:
In 1987 Orson Scott Card's "Speaker for the Dead," beat out William Gibson's "Count Zero" and Margaret Atwood's "The Handmaid's Tale."
In 1988, Pat Murphy's "The Falling Woman" won over Greg Bear's "The Forge of God" and David Brin's "The Uplift War."
In 1989 Bujold's "Falling Free" (another installment of "The Vorkosigan Saga" series) won the award rather than "Great Sky River" by Gregory Benford, "Mona Lisa Overdrive" by William Gibson, "Red Prophet" by Orson Scott Card and "The Urth of the New Sun" by Gene Wolfe.