Are you looking for individual novels or series or some combination?
For example, the entire Star Wars books as a whole is arguably one of the best-selling franchises. The individual novels, on the other hand, still sell very good (enough to get on the NY Times Best-Seller lists), but perhaps not as a lot as some classics (not that a lot of SF classics sell very well).
If you want individual titles, I'd say Ender's Game and Dune are probably one of the best-sellers.
If you want a modern best-seller count, I'd check the USA Today's List (
http://books.usatoday.com/list/index?f=10,0,122). Why USA Today? Less editorializing on the best-seller list compared to the widely popular NY Times. (More on their metrics at
http://www.usatoday.com/life/books/b...oks-list_n.htm).
Right now, if you discount the fantasy titles, it's currently Farenheit 451 and Ender's Game. (Interesting to note both are fix-up novels.)