[Loser]

Series is a very open term.

Robotech (for nostalgia, the books more than the anglified series of three unrelated anime, but that works too)
Babylon 5
X-Files first few seasons (though I'd call that Weird-Fic more than Sci-Fi)
Nadesco
El Hazard (more Fantasy than Sci-Fi, really)
David Brin's uplift books

That's probably all I'd call 'favorite', there's plenty more I'd watch/read happily. And of course there are plenty of isolated works I'd rank high than any series, Starship Troopers, How Much for Just the Planet, many others.