Of all of Joss Whedon's projects, I come to the inevitable conclusion that Firefly had the most potential out of all of them. However, I will also say that because it was only two episodes (if you include the unreleased ones) longer than Buffy's very own first season, it did not have enough time to sink its roots into the proverbial soil of primetime television, and therefore, I regard it second only to Angel -- which in turn -- I place above Buffy itself (even though, lol, Buffy was my first true TV love). It's a weird vicious circle.
Truth be told, all three of the series had excellent lead characters. Buffy was the angst-filled teenager struggling with an epic, legendary, and frightfully demanding duty that ultimately dominated all aspects of her life, much as she hated it. Angel was once the most sadistic vampire to ever 'grace' the Earth, and he fights for redemption now, cursed with a soul, struggling with his newfound conscience. Malcolm was a veteran of a war he ultimately lost, a narcissist and a cynic, struggling with his odd sense of honor. I often akin him to James Spader's character from Boston Legal (it's not really a stretch if you watch both shows), Alan Shore.
Whedon did an ingenious job, portraying each of these characters. I also have to say that Firefly's ultimate lack of success was due primarily to Fox launching the second episode ahead of the first episode(s), thus entirely screwing with the establishment of the characters. That, and Whedon was already managing two other successful shows at the time.
Just my two cents.