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
Probably Star Wars. Grew up watching it as a kid and the world feels very fleshed out.