Greg's favorite author is David Brin, who combines excellent writing, excellent stories and excellent science into one amazing package. In addition to Earth, The Postman, and his fabulous Uplift series, check out the incredible short stories in The River of Time, particularly "Senses Three and Six." No doubt a great part of his success is due to Mark's influence (noted in the afterwords of several books).
A close second would have to be Daniel Keys Moran, whose Armageddon Blues, Emerald Eyes, The Long Run, and The Last Dancer are (supposedly, anyway) merely the first installment of a corpus that is audaciously envisioned to encompass some three dozen novels, multiple intersecting storylines, and several billion years of history (across multiple timelines!). Sadly, Moran sort of petered out after Dancer, and his works are now exceedingly difficult to come by, excellent though they are. Best bets: used-book stores (though the otherwise outstanding Uncle Hugo's is invariably cleaned out), Alibris, or the like. (Also, a word of warning: the unfortunately similar pseudonym Daniel Moran, used by Robert Vardeman for The Flame Key and one or two other books in the same series, is in no way related to Daniel Keys Moran, and neither are his books of anywhere close to comparable quality.)
Other favorite authors include Roger Zelazny (Amber series, Lord of Light, Doorways in the Sand, Roadmarks, various short-story collections); Steven Brust (Vlad/Jhereg and related Khaavren series, The Sun, The Moon and The Stars, and To Reign in Hell); Iain M. Banks (Use of Weapons, Against a Dark Background, Feersum Endjinn, and more to come); Orson Scott Card (Ender's Game, Speaker for the Dead, and the Tales of Alvin Maker series); Neal Stephenson (Snow Crash, The Diamond Age, and Cryptonomicon); Isaac Asimov (everything); and Timothy Zahn (Cobra series, Conquerors series, Star Wars series).
A recurrent theme through many of these books--particularly those of Brin, Zelazny and Brust--is a smart-mouthed main character (not that Greg identifies with such characters or anything). And a recurrent theme through all of them is plain, old-fashioned, good writing--from both technical and artistic standpoints. Zelazny and Brust, in particular, like to experiment with the forms of writing; they don't always succeed, but when they do there is nothing to compare with it. Brin's "Senses Three and Six" mentioned above is another example of such writing, condensed into a shining jewel of a story. Subtlety is the hallmark of these authors.
Card's novels are unusual for what can only be described as "storytelling." Better than any other fantasy or science fiction author, living or dead, Orson Scott Card practices the ancient art of the storyteller. Other authors may have better stories or may be better writers, but none tells a tale like Card; his writing evokes the oral tradition of the wandering folklorist. Seventh Son not only is a perfect example of this sometimes-folksy-but-never-overdone style of writing, but it also includes a character who is its (fictional) living embodiment.
Stephenson's work is breathtaking in its incredible attention to detail, specifically with regard to the logical extension of all aspects of future development--not merely technological but also social, economic, political, ecological, biological and behavioral. Brin's Earth, notable for its vision of future ecological and sociopolitical trends, comes closest to matching the scope of Stephenson's books. But the latter go beyond "mere" breadth and actually achieve definitive stature. For virtual reality, it is Snow Crash by which all others--even William Gibson's groundbreaking Neuromancer--are measured (indeed, its influence was strongly felt even in the nonfictional computer industry during the 1990s); for nanotechnology, it is The Diamond Age.
Also more than worthy of mention are fantasy authors Emma Bull (War for the Oaks), Patricia McKillip (Riddlemaster of Hed trilogy), and Barbara Hambly (Dog Wizard trilogy; Walls of Air trilogy; and probably others if he could just remember them, but some clown borrowed almost his entire collection without ever returning it, and now Greg doesn't remember who did the foul deed). Among old-timers, hard-SF writers Arthur C. Clarke and Larry Niven rank right up there, too.
On a slightly different note, quite a few authors have come to Greg's attention through a single novel--not necessarily because that's all they've written, but because Greg has far less time for reading than he used to, and by the time he finds a good one, often the companion volumes are out of print or otherwise difficult to find. (And yes, sometimes the author never wrote anything else of note...) Here are a few of the standouts [more to come Real Soon Now]:
Finally, Greg blushes to mention that he wrote a book, too. It's not nearly as entertaining as any of those just mentioned, however...
Greg's least favorite author on the entire planet is Piers Anthony, a pathetic hack who couldn't write his way out of a paper bag. Some of Anthony's concepts were actually pretty good--the first couple of Xanth books (there are now dozens of them, which says something in itself), the first two or three Phaze books, and Death as described in On a Pale Horse--but even these were tainted by the author's condescending tone, apparent misogyny, incredible talent for beating a dead horse completely underground (see Xanth, above), and just plain bad writing. Everything else he's written is the fantasy equivalent of Harlequin romances...or Microsoft software.
Greg also has a difficult time reading anything by Robert Heinlein anymore, not because he's a bad writer or one who just churns out the same old crap, but because he tends to preach--about military service, about honor, about love, and about life in general. Time Enough for Love is a masterpiece of a novel, but it is nevertheless clear that Lazarus Long is little more than a mouthpiece for all of Heinlein's most strongly held opinions and cute sayings. It gets old after a while.
A young author with the same problem but far worse is Gael Baudino. Her Gossamer Axe was a very good (first?) novel, despite its obvious similarity to Emma Bull's wondrous War for the Oaks. But it was slightly marred by its preachiness and by the revelation in the afterword that it was largely autobiographical. That could be excused, of course, but the fact is that her subsequent novels were more of the same and then some. Our hero can stomach one thinly disguised treatise on Wiccan Goddess-worship, but four of them is enough to induce projectile vomiting. Shelve 'em in the New Age section.
Special mention goes to Philip Jose Farmer, whose interminable Riverworld series showed a lot of early promise but, like the river of its title, meandered all over the place without actually getting anywhere. Greg is still wondering whether anything ever happened in the fourth novel or if the whole thing was intended as some devious form of mental torture. This was one of the few novels he couldn't finish.