These figures are taken from the Wikipedia page on the subject.
Dickens' A Tale of Two Cities is the biggest selling novel of all time for which a figure is available - I include it to bound the info-graphic. It's not fantasy. It's not the best of books, it's not the worst of books. I did enjoy it though.
Figures for Harry Potter & the Philosopher's Stone and A Game of Thrones are lower bounds. The Harry Potter figure is from the Wikipedia list but for the 7th book. Clearly the 1st book sold more than the 7th but there is no figure given. The Game of Thrones figure is that for the 5 books in the series (taken from the Wertzone) and divided by 5 - this is a minimum.
click image to enlarge
Obviously I could only include a few titles, but the fact is that very few fantasy titles are on the list. A Game of Thrones didn't have enough sales declared to make the list (need 10 million+ for an individual title).
Anyway, the bottomline is that if you could just nip out and buy 199.5 million copies of Prince of Thorns I could have the biggest selling novel in history. Thanks!
For even more context. Here's the diagram scaled against Agatha Christie's sales of crime stories. Christie has sold more books that anyone else with the exception of Shakespeare. The bible outsells both.
Note, this is the lower estimate for Christie, it could be double this.