I think the first 4 or 5 seasons of the x files holds up pretty well. That spawned a whole slew of copy cat, crappy alien shows.
Personally, I think the first few seasons of the x files was actually the first decent fantasy/horror television show.
I still dont think any decent scifi television has been made. I know a million scifi nerds would throw out contenders but the fact of the matter is that they all sucked (The Expanse is probably the best and that still looks/sounds/plays like it was produced by high school kids). When someone does a miniseries of The Sparrow or Dune with the writing and production quality of True Detective or Deadwood Ill change my opinion. Science fiction deserves to get it's own version of Game of Thrones.