The Guardian has an interesting topic asking if novelists' most famous works are their best. Their list examines literary fiction, so I thought it would be interesting to look at the SFF field.
Most famous work: The Chronicles of Thomas Covenant, the Unbeliever.
Best work: The Gap Series.
Stephen Donaldson became one of the founders of the modern epic fantasy movement in 1977 with Lord Foul's Bane, the first in his Chronicles of Thomas Covenant series (originally a trilogy but now nine books, with one more to come). It's his biggest-selling and most famous work, and certainly a laudable attempt to bring more adult and literary techniques to bear on the subgenre, but for me it's outclassed by The Gap Series. The Gap starts with a short, lyrical novella about perspective and truth before suddenly exploding into a colossal SF reworking of Wagner's Ring Cycle, filled with complex clashes of characters and cultures and delicious political intrigue. The best thing Donaldson's written. No-one bought it though, hence the return to the Covenant books.
Arthur C. Clarke
Most famous work: 2001: A Space Odyssey
Best work: Childhood's End
Thanks to the era-defining movie version, 2001 is easily Arthur C. Clarke's best-known work. However, the novel is actually among his less impressive books, rich in atmosphere but lacking in overall incident. In fact, purely on a novel basis, I'd rate its sequel 2010: Odyssey Two as being a much stronger book. Childhood's End, on the other hand, is for its day visionary, transcendent and mind-blowing, with a stunning finale marking the end of the human race (or rather our current stage of existence) and doing so in an unforgettable way.
Most famous work: The Robots/Foundation universe
Best work: Nightfall
Picking out Asimov's most famous work would have probably involved some elaborate Twitter polling on whether I, Robot or Foundation was up there, but fortunately Asimov solved this problem by, somewhat unconvincingly, retconning them into the same universe. However, for me his strongest work is the short story Nightfall, in which some scientists on a planet with six suns in its sky discover that for the first time in recorded history there's going to be an eclipse with only one sun visible, meaning that for the first time in thousands of years, night will fall. A simple story based on a rudimentary scientific premise with tremendous ramifications for society and the individual people involved. Terrific and, rather unlike the 15-book Robots/Foundation/Empire universe, straight to the point. The novel version (with Robert Silverberg) is interesting but lacks the short story's punch.
Most famous work: The Monarchies of God
Best work: A Different Kingdom
Possibly a bit of a stretch, given that Paul Kearney is still chronically under-read and even the splendid Monarchies of God fantasy series is still reasonably obscure (though now rising, with the recent reissuing of the series in two omnibus volumes). However, when people talk about Kearney, it's his epic fantasy series which are always mentioned (Monarchies, Sea-Beggars and the current Macht trilogy). His finest work for me is A Different Kingdom, which starts out as a semi-autobiographical novel about growing up in rural Northern Ireland before fantastical events start taking place. The protagonist finds himself drawn into the woods neighbouring his farm, and finds a different world waiting. Rich and mythic, A Different Kingdom can be summed up as an Irish Mythago Wood, whilst also being totally different to Holdstock's masterwork. Overdue a re-release.
Most famous work: The Prestige
Best work: The Separation
Thanks to Chris Nolan, Priest's very fine novel about battling 19th Century magicians is now quite well-known. However, for me his finest novel remains his most recent, The Separation. Almost killed at birth by uncaring publishers, the book was rescued by Gollancz and is a staggering achievement. A pair of twins become embroiled in the Second World War, but not necessarily the war we are familiar with. With dizzying shifts in perspective and constant evolution of the backstory, the book is mind-blowing and will invite constant re-reads and analyses to tease out its secrets.
George R.R. Martin
Most famous work: A Game of Thrones
Best work: Fevre Dream
After HBO's great adaptation of A Game of Thrones, it's easily currently Martin's best-known work. But it's not his best. In the context of A Song of Ice and Fire itself, my favourite piece of writing is The Hedge Knight, the novella set 90 years before the novels and introducing the adventures of Ser Duncan the Tall and his squire, which uses Westeros' rich backdrop for a much simpler, much more concise story than the expansive novels. However, even beyond that, Martin's 1982 horror novel Fevre Dream has something going for it his fantasies don't (so far): an ending. Martin's story about vampires on the Mississippi trying to develop a drug to wean them off blood is dark, gripping, rich in atmosphere and tragic. Someone needs to make the movie (and cast Ron 'Rodrik Cassel' Donachie as Abner Marsh!) yesterday. And resist the urge to turn it into a True Blood prequel.
Most famous work: Memory, Sorrow and Thorn
Best work: Otherland
Tad Williams exploded onto the scene with his Memory, Sorrow and Thorn trilogy in the late 1980s, which was hugely influential and set the scene for many of the authors who followed. Whilst overall a fine work, the trilogy skews very much to the traditional and has a somewhat annoying ending. Otherland, on the other hand, is much more original, being a cyberpunk-fantasy crossbreed. Williams uses the SF backdrop to explore a lot of excellent and fantastical ideas. Whilst the saga is still too long, its episodic structure makes it fun to read and the premise makes for a rich vein of story ideas which could sustain entire series (the reverse-Aztec invasion of Spain is particularly interesting). Overall, a strong series which has now spawned an MMORPG and a particularly large fanbase in Germany.
Any other thoughts and suggestions? On Twitter I've already had Gene Wolfe nominated, for Soldier of the Mist over The Book of the New Sun.