Babylon 5 is a science fiction TV series created by Joseph Michael Straczynski. Overall it has a similar concept to Star Trek, particularly Deep Space Nine. But in practice it is quite unique.
While Star Trek can be overly preachy for some I believe Babylon 5 asks far more questions then it tries to answer. Star Trek can also be accused of being overly human centric and pro human, I do not think this can be said (at least very strongly) of Babylon 5. One thing they do share in common are quite a few actors, including Lwaxana Troi and Pavel Chekov.
The Babylon 5 universe spans two TV series (Babylon 5, and Crusade), 7 movies (The Gathering, In the Beginning, Thirdspace, The River of Souls, A Call to Arms, The Legend of the Rangers, Voices in the Dark), numerous novels and even a few games; And manages to be a very complex, interesting, and diverse universe.
I really love the different races and groups in the Babylon 5 universe. While all of them are interesting I found two of them particularly interesting, the Telepaths and the Technomages. In Babylon 5 telepaths come from almost all races, but the ones shown are almost all Human. They are often looked upon with fear and hatred by non telepathic humans and are derogatorily called teeps. Because of this fear the Psi Corps was formed to control the human telepaths. Technomages are a multi raced group of cyborg technicians, they employ technology to simulate magic. They are very well done and have many interesting tricks while never doing anything outrageous and unexplained.
Personally, I am a huge fan of complex fictional universes that take many books/films/TV series to outline properly. And while I have seen many, Babylon 5 is my overall favourite universe of all time.