Good lord, ever?
My main issue is that the shows that have produced my favourite hours of TV ever have degraded somewhat as they went along - Stargate SG-1, ER, Buffy and Battlestar Galactica (Noughties version) would be the most obvious examples of this.
Firefly didn't have a chance to degrade so is up there almost by default; DS9 and Angel are the lovely exceptions to the rule in that they got better as they went along.
If I had to pick just one, it would probably have to be Stargate SG-1, with the caveat that I mean the first 6 seasons or so, not the dross with the cast of Farscape. I love mythology in my shows, I'm a dreamer - and the mythology of the Goa'uld, the Ancients, and the four race alliance just hooked me completely. I just pretend they didn't cock it all up later when we actually met the Ancients. The Fifth Race remains one of my favourite episodes of television ever.