Two fantasy writers pretty much defined my childhood - David Eddings, who did the
Belgariad/Malloreon and the
Elenium/Tamuli, and Joe Dever, who wrote the Lone Wolf gamebooks. Dever only wrote one franchise and I even met him once, as opposed to Eddings, who wrote multiple franchises and turned out to be
a convicted child abuser, so I'll have to give the crown to Dever.
For sci-fi, it has to be
Star Trek by a country mile, though I'm quite fond of the Old Republic era of
Star Wars. Of course, Star Wars isn't sci-fi, and Trek is very soft sci-fi, but still.