I'm a big fan of stargate. It's a great series with a lot of back story fictional languages and fictional cultures. But the reason why I really like it is that it's set in modern times (starts late 90s) and still is a science fiction show. They also tie in conspiracy theories and ancient alien stuff which is really neat. It's fairly unique I think for a science fiction series. it's also totally true
Best of all has to be Trek for me. Just the sheer size and depth of it from decades of series and films. I do like star wars, Dune, farscape, Firefly and Alien a lot to (i dont like the predator part of the last that much though) Aside from those franchises I really like games workshop's 40k universe to. The tabletop game has been around for decades and there is so much depth in storylines and history.