This and BSG are p. much my favourite sci-fi shows of the last ten years. This is objectively way better than BSG, though. I like BSG in spite of its flaws. I'd be hard-pressed to identify flaws in Firefly.
Of shows I've seen through to the end, only Star Trek: TNG comes close.
(That said, there are shows I've been desperately neglecting. I've seen a few episodes of Farscape, which is excellent so far, and I need to see Babylon 5).
But yeah, a more engaging cast of characters on television I'd be hard-pressed to find, and it's really terrible that Whedon didn't get to put them through all the character development he had planned for them.