Personally, I believe that while the show was not as godlike as the diehard Whedon fans believe, it also was a world apart from Whedon's previous work.
In other words, Firefly was where Whedon was /beginning/ to get serious and was consciously struggling to raise himself out of the "hack" category, putting the good parts of his abilities to use. People who dismiss the show as being "just like all his other stuff" are reading in what isn't there; not all the characters are his stock characters, they actually have different voices and personalities, and the universe he was setting up had much more nuance and was damned intriguing.
While it was clear that Firefly represented a maturation in his capabilities as a creator, you aren't giving his previous work enough credit. Buffy and Angel's characters were just as diverse and interesting, and had voices just as distinct. Only the setting was different.
I am absolutely in favor of the statement that Firefly is nothing like what most people's preconceptions of Buffy
would lead them to suspect. However, since most people's preconceptions of Buffy
are inevitably wrong once they actually see the things, that doesn't translate to a statement about the shows' relative merits.
There's no question in my mind that, had it continued, Firefly would have surpassed them both. But as it stands, I still think Angel
is the strongest of the three.