It's really, really hard to vote on things like these, because none of his shows suck. They're all brilliant. But, if I had to choose I'd pick Firefly. It's not what comes to mind immediately when I think Whedon (in fact it's Buffy), but I feel that Firefly is Joss' masterpiece. It was amazing from the get go. Literally, the first scene of "Serenity," with Mal, Zoe, etc. being abandoned at Serenity Valley.
So, yes. Firefly = best Whedonverse show. Buffy = my favorite Whedonverse show.