Favorite F&SF Series
There is no restriction as to whether a series contains directly-related novels (that is, one long story spread over multiple books) or independent stories set in the same universe. As the list shows, the majority of series I prefer (13) fall into the latter category.
My prejudice towards science fiction rather than fantasy shows, as only 4 fantasy series made the list, and all are on the lower half. Before anybody starts screaming at me, I have not read most of the "major" fantasy series by authors such as George R.R. Martin, Robert Jordan, Stephen Donaldson, etc., so they did not make the list from lack of familiarity rather than any specific opinions I have towards them. This is not meant to be a definitive list, merely my personal preferences.
And, yes, only three women authors are on the list, but women tend to write fantasy, and I prefer science fiction, so that is a natural reflection of my interests.
1 / Darkover / Marion Zimmer Bradley
2 / Alliance/Union / C.J. Cherryh
3 / Alex Benedict / Jack McDevitt
4 / Mars / Kim Stanley Robinson
5 / Galactic Cluster / Jack Vance
6 / Pern / Anne McCaffrey
7 / Riverworld / Philip José Farmer
8 / Hyperion / Dan Simmons
9 / Galactic North / Alastair Reynolds
10 / Polesotechnic League / Poul Anderson
11 / Earthsea / Ursula K Le Guin
12 / Nevérÿona / Samuel R. Delany
13 / Galactic Center / Gregory Benford
14 / Ender / Orson Scott Card
15 / Gateway / Frederik Pohl
16 / Sector General / James White
17 / Known Space / Larry Niven
18 / Majipoor / Robert Silverberg
19 / Amber / Roger Zelazny
20 / Book of the Long Sun / Gene Wolfe