I'd say Firefly is most likely, mostly because the other options (that I'm familiar with) usually have several glaring faults even in their internal science, and when trying to apply the real world everything just goes kablonkers. At least in Firefly most of it makes sense, there's still the bloody reavers, and some other nonsense, but most of it works, and it seems rather nice too.
I'd suspect that or something like A miracle of science (but without the mad scientists) would be most likely to come to pass. The science mostly checks out, the settings are still human, and evolved, they're not just today with a new decor (Star Trek) or going with rule of cool and ignoring laws of physics, causality and similar effects (Star Wars, Stargate, 40K).
I can't make any predictions about alien species though, I doubt they'd function as presented in fiction, especially the four I've mentioned since those just make NO sense whatsoever. I'd like to think we'd be able to establish peaceful relations. I doubt there'd ever be an interstellar or galactic war. The logistics, or logic behind it just doesn't check out. If we ever become a spacefaring race it won't just be our world, in it's current state, but on a larger scale. I doubt there'll be space pirates, smugglers, crime lords, police vessels or that much trade flying around in space. It just doesn't make sense that society wouldn't have progressed, or that it'd viable, even if people wanted to. With the technology we'd have available, most such problems should be removed. The only reason would be some sort of selective technological regression, like in Firefly, or Freelancer, following a cataclysm of illogical proportions.
