I'll echo many others here.
The great: Tolkien, Lieber, Moorcock, Vance, Howard, Le Guin
The good: Saberhagen, Drake, Brooks, Eddings, Jordan, Goodkind, Zelazny, Niven
Adequate: Salvatore, Weis & Hickman, Asprin, Anthony
Pulp fiction: Greenwod; all the FR/D&D/Greyhawk etc novels
Of course, there's a lot of science fantasy out there (McCaffrey, Saberhagen could fit here, anything Star Wars ...).
For the names some may not recognize:
Fred Saberhagen writes a lot of science fiction, but his epic Changeling Earth, which includes Empire of the East and the Book of Swords series is really a science/fantasy classic.
David Drake is well known as a military science fiction writer, but he has a four-volume series out now that begins with Lord of the Isles that is based in classical literature and would make a great D&D setting.
Larry Niven is primarily a science fiction writer, but The Magic Goes Away and its related stories are an interesting take on assumptions about magic.
Even if you don't like Eddings, if you're a DM with a homebrew world you should read The Rivan Codex, which explains in detail how Eddings created his fantasy world, the societies, and the literary conventions behind the setting and characters. I don't know of many authors who (while they are alive) publish the background notes for their work and explain why and how they did what they did.