I think Joss Whedon's Firefly (and its spin-off movie Serenity) would probably be the number one fan-pick if they were told any cancelled franchise could rise from the ashes again. When Whedon became arguably the biggest director out there after the massive success of Marvel's The Avengers, a little spark of hope was ignited -- but unfortunately since then there's been no word, and Whedon himself hasn't given any indication that it's part of his plans. Hitfix put the question to Mal Reynolds himself Nathan Fillion, and here's what he had to say:
"That was a project that lived for so short a time, and such a long time ago, I would look at that as an impossibility. Joss Whedon made a movie of it -- something I also considered an impossibility. So I look at that and think 'where is the hope?' but knowing Joss Whedon...I try not to underestimate that man."
Hmmmm. I guess you never know what could happen down the line, but I wouldn't hold my breath!