I would have to say Deep Space Nine.
Started off weak, but when they introduced the Dominion and had the war between them and the Federation that's when it became entertaining.
Babylon Five was okay but I did not find it too riveting. I think just the whole design of the show did not appeal to me. From how Babylon Five was designed; to how the computer generated space battles looked. (I have to admit; I always enjoy models over computer graphics.)
Anybody been watching Enterprise?