Sorry, but the best Sci-fi series ever is Red Dwarf..
If you prefer 'straight' sci-fi then I think the general situation is analagous to that with sci-fi movies. The majority are so cringe-makingly awful that a merely bad one is a relief. And a good one seems like the best thing you ever saw. I'm saying this as someone who actually LIKES sci-fi - it's a shame that the best books so rarely get translated.
I really haven't seen enough of Farscape to lump it in with that generalisation. But I was astonished that a movie as poor as 'Stargate' actually got a spinoff series. I thought it was straight-to-video garbage, but I guess a significant number of people disagreed with that