NOT your personal favorites (subjective)! The best-sellers (objective). Group book series together. Anyone know? Educated guesses are OK, but documented answers are preferred.
For example, I'm guessing that, for sci-fi, the top 3 are the Dune series, the Foundation series, and the Ender series. Or maybe the Space Odyssey series? Or, heck, maybe the Star Wars or Star Trek or Honor Harrington series? See, I don't know!
[For admins: I think this question should have all the genres as tags, but only 5 are allowed. I could make it 13 different questions, one for each genre, but that seems like gaming the system to gain rep. Also seems not as useful. But whatever you want is fine with me. If this question is handled elsewhere, then let me know, and delete this question. Thanks.]