I'd have chosen Firefly, I never get tired of it, and I just love it.
So many other amazing shows though, a few already mentioned in other comments, like BSG would be a close second to Firefly.
But there are some shows that haven't been mentioned and I think they deserve it just as much. And because having to choose only one would make me regret losing these other ones, here they are:
Breaking Bad, it absolutely fascinates me and I just can't get over the character development and the writing and acting from the whole cast.
And probably the newest show that for me is on the very top, and actually I must admit it might have gone past the other ones (except firefly), Arcane. Everything about it, I love. The writing, the animation, the music, the characters, the world building, the character growth, ...
If I choose Firefly, I'd regret not choosing Arcane, but I just can't not choose firefly.