(I *like* Buffy. I *like* Xena. It's just that I'd like to see a female, non-buff,non-gorgeous, normally attired nerd who isn't either a dork or a best friend. Just once in a while.)
There's Rosie O'Donnell, but she's a talk show host. When she acts, it's back to the best friend role. Seriously, in America, Drew Carey's about the only male who fits the "non-buff,non-gorgeous, normally attired nerd who isn't either a dork or a best friend" description. So it's not so much a matter of sexism as sexyism-- unfortunately, with the visual qualities of televesion and movies, there's not a great chance of overcoming the camera's love of eye candy. One reason I tend to like British entertainment a bit more is that the girls aren't all size 0 in dresses; there's a closer approximation to reality in the variety of ages, sizes, and style than modern Hollywood shows.