A list of the top 19 (why not 20?) SF series published on the Popcrunch web site is an interesting list – although I have no idea what the Popcrunch site is all about.
Of the 19 I have read 11 of the series and think they were place on the list pretty much the way I would have ranked them. The 8 I haven't read I obviously have no opinion..
The top 2 - Asimov's Foundation series and Niven's Ringworld trilogy - Clarke's 2001 series and Card's Enders series bring up the tail end.