You won’t find many people to go against Firefly. Do try to watch them in the correct order, though, rather than the broadcast order.
Babylon 5 is excellent too, and a lot of later programmes have blatantly stolen a lot of ideas from them. Was very big on long-running story arcs, which a lot of later programmes have done too. In fact I blame them for the “we don’t have episodes” trend in modern TV which produced plot-free, constant-cliff-hanger rubbish in which nothing ever happens, like Lost.
I wouldn’t bother with Andromeda, not bad and with some very good episodes, but nothing special. Same goes for Earth: Final Conflict, and in both the fifth series especially should be avoided.
X-Files and Stargate both have a lot of episodes, some of which are very run-of-the-mill, but the good ones are very good indeed. X-Files myth-arc episodes best avoided, the more light-hearted ones are the ones to watch.
Obviously, Star Trek is the one all the others get judged against, and is the behemoth which dominates the whole genre.
nBSG is like Marmite, you either love it or you hate it.
Quantum Leap’s alright, too, good to watch but nothing special. Looks a bit dated, too, and has the old-fashioned idea of self-contained episodes. Certainly the best of the bog-standard sci-fis there were so many of during the 90s, back before the schedules were filled up with reality TV and all that cheap, mass, rubbish. Other honourable mentions in that category go to Space Above and Beyond, Dark Skies, First Wave, Earth 2, probably Earth Final Conflict, Sliders, that sort of thing.
I’d also like the recommend some English sci-fi, some of it quite different from your space-opera type stuff, specifically “Jekyll”, “Invasion Earth”, “Primeval”, “Sea of Souls”, and, of course, “Red Dwarf”.