Order will be hard. Worse is I have since discovered
https://www.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g and watch most of what he does, which in turn means where I once might have been able to shut off science boy in my head then I have no chance today.
Anyway.
I think The Expanse will be here for the long haul for me. The
books were probably better (more time to breathe as it were, but at the same time some of the choices made to the show would have been welcome if seen in the books) but that does not mean the show is/was bad.
Red Dwarf. It is a comedy show but I think it is probably what pointed me at hard sci fi more than most, though or hard magic in some fantasy books I was reading at the same time.
Starship troopers the series. CGI thing aimed at kids sure but I used to wake up early every morning to watch it, and still would.
I have not seen the new Battlestar Galactica since it finished, and never saw the follow up prequel thing, but I reckon a lot of what we see nowadays is owed to it. You could go back further and I am sure someone would say a lot owes to Farscape or some more obscure European efforts (I mention Lexx in a moment), and while quite probably true this did it as far as most TV types are concerned.
I should note that I am probably an utter failure as a nerd and never really properly sat down and watched Babylon 5. If I was going to do Star Trek then Voyager would be it but I am indifferent there. I can't include Lexx in the list above (naturally the German/not American cuts and uncensored versions thereof) but want to note it in passing here at least. I don't know how well Stargate would have aged here (
https://gbatemp.net/threads/stargate-viewing-order.79173/ ), and people tell me I should one day finish Universe as it actually got better after everybody stopped watching it.
I am also pointedly ignoring anime in this as several of those would vie for a position here and I don't actually care about anime all that much. Cowboy Bebop then probably being the fifth on the list, but even more middling stuff like Knights of Sidonia would want to feature in the discussion.
Failing that I do like the occasional bit of sci fi that will not last the ages -- in recent times sci fi TV network, or syfy as they are known these days, did some nice stuff and I actually got on well with the likes of Killjoys and Dark Matter. Probably won't remember much of them in 20 years the same as I don't remember much of Space Precinct today (I had to find that with a search of the description rather than the name) but I would watch them week on week, which is more than could be said for some things. I will also include Firefly in that list -- I can understand the appeal if you had not had all the nice European and Japanese efforts but watching all it a few years after the film, and going in cold other than some people on the internet going all fanboy enough for me to note the name,... yeah. Mutant X would be another example of one in between some of those.
Agents of S.H.I.E.L.D.? I caught the occasional episode as it came out on the same days as some things I was properly watching but it never clicked. If I have to do something here then if Mutant X does not count then Iron Man animated comic for extremis.