As was previously said, it depends on what you are looking for.
Stargate SG-1 is similar to Star Trek in some regards, in that they are exploring new worlds and it focuses on some of the same ethical issues brought up in regards to meeting new civilizations. It is the most humorous of the shows you mentioned and with four main characters (five if you count Hammond, which I excluded simply because he doesn't go with them on missions and, as such, doesn't get the camera time the main four get), which allows for some good characterization and the interplay between the characters is really great. Like most shows its first season is not as strong as later seasons.
Babylon 5 is among my favorite TV shows because, while it starts out somewhat similar to Star Trek, it really hits its stride in the second and third seasons. As has been endlessly debated, it does have some similarities to Deep Space 9 if you liked that show, but it truly is its own show. If you like space-oriented sci-fi shows with arcs, then you might really like B5.
As far as Battlestar Galactica goes, as a fan of the show, I think it's fair to say that the mini-series and the first two episodes, proper, of the series are in a class by themselves. You're right in that it really doesn't get better (if you meant that you want a show that gets better as you go along), though I think there are many episodes throughout it's run that equal the mini, "33," and "Water," in terms of quality. The rest is good to great (with some exceptions).
As for shows others mentioned:
I'm not as huge a fan of firefly as some on here, but I did start re-watching it yesterday. It's definitely a good show and Whedon does amazing things with characterization. However if you are looking for something similar to Star Trek, this may not be the one for you. If you are interested in a unique take on sci-fi that avoids some of the tired sci-fi tropes and favors an interesting mix of a futuristic show and an Old West series/movie, give it a shot.
I'm a big fan of Doctor Who, but it's hard to describe. The first four seasons since the show came back in 2005 are available on Netflix, and hopefully the fifth season will be available soon. I was hooked by the first two episodes, "Rose" and "The End of the World," and if you like them, then it definitely does get better, especially in the episodes written by Stephen Moffat, who, as of the fifth season, took over as the showrunner.
I'd add seaQuest DSV. It's true that the first season is the best season, though I did like some of what they attempted in the second and later, third season, it almost becomes a different show each season. The first season is a good mix of the usual sci-fi tropes, though set in the nearer future than Star Trek. I think it's fair to say that if you enjoyed TNG, you'll enjoy at least the first season of seaQuest, if not the latter seasons, as well.
On a side note, we've been subscribers to Netflix since the summer and it's great. With the "Watch Instantly" feature and all of these shows available to you, I'd suggest just picking one and jumping right into it. If you don't find you like it, go try another show. That's the great thing, you are only limited by your free time and your taste in shows.
