Citizen Kane is famous for a bunch of different reasons, including advancing the state of the art of cinema and being basically a flawless masterpiece. But probably the thing that stands out the most today is its psychological depth, in that to this day it's arguably still the best character study ever done.

Firefly doesn't have that sort of psychological depth, but it does have a lot of moral ambiguity, something that's virtually non-existant in CK. That's something that would have gone a lot deeper had the show gotten another two or three seasons, but they never got that chance. It also has some of the most clever writing of any TV show, up there with only Arrested Development really. Cleverness might not be as prized by critics as authentic character writing, but it's still at the top of its form nonetheless. The main downside of the show was that it was already starting to get a bit formulaic toward the end of season one when they canceled it. In any event, I don't think you could ever put Firefly above Citizen Kane, but certainly I think it pushed the boundaries of its genre in a way that is rare.

The Avengers, on the other hand, has virtually no redeeming value whatsoever except for Joss Whedon's clever writing during the second half and the amazing effects during the NYC fight scene.

Moral ambiguity is something that passes in and out of fashion, and it's something that Firefly never really delivered on. Where Firefly really shone was as a prototype for the naturalistic science fiction approach that Battlestar Galactica mastered.

Yeah, it's unfortunate that Firefly never got the chance to deliver there. That said, if you pit the first season of Firefly against just the first quarter of Citizen Kane, which is really more fair in some ways, I'd go with Firefly any day.

I don't think "the chance" is what it lacked. Firefly was relentlessly light and couldn't have sustained the darkness necessary for real moral ambiguity to work.