According to Wikipedia:
- J.K. Rowling's Harry Potter (7 volumes, 3 supplements) - 450m
- Star Wars (300+ volumes) - 160m (missed this the first time around, whether or not it is fantasy or SF, I'll leave to you...)
- J.R.R. Tolkien's The Lord of the Rings - 150m (Wikipedia classifies it as a single volume)
- C.S. Lewis' Chronicles of Narnia (7 volumes) - 120m (The Lion, the Witch and the Wardrobe alone - 85m)
- Stephanie Meyer's Twilight (4 novels, 1 novella, 1 guide) - 116m
- J.R.R. Tolkien's The Hobbit - 100m (Not a series - unless you're Peter Jackson)
- Anne Rice's Vampire Chronicles (12 volumes) - 80m
- E.L. James' 50 Shades of Gray trilogy (3 volumes) - 70m (For comparative purposes)
- Tim LaHaye and Jerry Jenkins' Left Behind (16 volumes) - 65m
- Terry Pratchett's Discworld (39 volumes) - 55m (65m according to the author's note in Unseen Academicals)
- Edgar Rice Burrough's Tarzan (26 volumes) - 50m
- Suzanne Collins' Hunger Games (3 volumes) - 50m [The Hunger Games alone - 23m)
- Robert Jordan and Brandon Sanderson's Wheel of Time (14 volumes) - 44m
- Christopher Paolini's Inheritance Cycle (4 volumes) - 33m [40m now]
- Stephen King's Dark Tower (8 volumes) - 30m
- Terry Goodkind's Sword of Truth (12 volumes) - 25m
- Terry Brooks' Shannara (20 volumes) - 21m
- Eoin Colfer's Artemis Fowl (20 volumes) - 21m
- Isaac Asimov's Foundation (3 volumes) - 20m
- Brian Jacques' Redwall (22 volumes) - 20m
- Dragonlance (150+ volumes) - 20m
- Charlaine Harris' Southern Vampire Hootenanny (21 volumes) - 20m
- Douglas Adam's (and Eoin Colfer) Hitchhiker's Guide (6 volumes) - 16m
- Raymond Feist's Riftwar (25 volumes) - 15m
- George R.R. Martin's A Song of Ice and Fire (5 volumes) - 15m
Random thunks below...
Most of these figures are rubbish. Sales figures in general are dodgy as hell - accurate ones would depend on a collaboration between publishers, Nielsen and Amazon - a utopian data hegemony that seems about as likely as my spontanously turning into a flamingo. In this case, glancing through the footnotes, it looks like enthusiastic Wikitypes have gleaned them from studying magazines and press releases. There are also some absences - Richard S. Prather's Shell Scott sold over 40 million copies, for example. (Which is, amongst other things, a lesson in literary mortality.) And, to top it off, this also isn't the most frequently updated page (the GRRM numbers, for example, are almost a year old).
But as an indicative list, we can more or less assume that all the numbers are equally screwed, and, even on top of that, taking into account that these are all extreme outliers... it is pretty damn interesting.
[Editor's note to add... I included all the science fiction books as well, as there weren't many of them.]
For one, I've always thought that SF vs Fantasy is a cyclical thing (my pet theory is actually economic: recession/fantasy, boom/SF). This may still be true (see all the points above), but, as a whole, fantasy has been thumping SF pretty routinely since, well... ever.
As always, there's also a lesson about not being a dick about other genres. It is great to see fantasy and science fiction represent on these lists. But it is also impressive to see erotica, manga, thrillers, young adult all showing up as well.. You know, no matter how smug you feel about trashing Fifty Shades of Gray, it has outsold A Song of Ice and Fire by, er, 55 million copies.