Babylon 5, hands-down. Though I'll admit to a soft spot for TNG and Voyager grew on me after a while, Babylon 5 will always be my preferred space opera of choice. I enjoyed that it was a little grittier and rougher than the Treks, but interwove philosophy and humanism into the mix of the story arc.
Babylon 5 was easily the best Scifi to date. With realistic physics, and politics, excellent actors, great stories and an overall story arc that was just awesome.
They are working on a remake of the show right now. It's too bad they couldn't just update the effects from the old show. I know they remastered it, but it still looks dated now since they were not using models.