I mean, "BUFFY IS THE BEST" isn't much of an in-depth analysis, and in turn, my "Angel & Firefly are the best" isn't super-well-explained either.
BUT IF YOU WANT A RANKING THREAD
Whedon has a relatively short attention span, really. And if a project doesn't have 100% of his focus, the quality will diminish rapidly depending on who is left in charge.
Buffy's first season is rough, which is to expected because he wasn't used to running a show. But seasons 2 & 3 were pretty goddamned good. 4? Not so much. 5? Came back strong.
6&7 can take a fuckin walk, for the most part.
Over the course of Buffy's run, even in its best seasons, the quality was sorta/kinda inconsistent. This is to be expected when you hang your show on Sarah Michelle Gellar's acting.
But when Whedon turned his full attention to Angel, not only did Buffy suffer a little, but Angel took off. Angel started at a higher level of quality, and maintained it for longer. Boreanaz was a better lead, and the support was better still, and when Whedon left THAT show to wrap up Buffy and start Firefly, the quality didn't dip as strongly.
And then there's Firefly, which had the benefit of Whedon being a pro at showrunning, and knowing EXACTLY what he wanted to do from jump, as opposed to both Buffy and Angel, where it took a little time to find a groove. Firefly being canceled in the middle of a season and ending with a movie was probably best case scenario, honestly. No time to burn out, get distracted, or diluted.
Buffy: Much more groundbreaking, way more uneven.
Angel: Much more consistent + higher highs (best ending, too)
Firefly: Best execution, most assured storytelling.
I honestly think the deification of King Joss in the mid 2000s, the resurrection of his dead show, and the constant calls to throw him at literally ANY "geek" property (and his finally being gifted one, and it being the fuckin' AVENGERS) dulled him.