Lois Griffin is far and away the sexiest animated woman, if you go by behavior and not just looks. The wild and kinky things she's done at various times have no analog in any other mainstream venue.
Mythic curves, etherial sheen--courtesy a CGI makeover--and a teasing drizzling in liquid gold. What more could anyone ask? Built-in high heels? Check!
I pick Kim Possible. Only because I have a friend who looks a LOT like her, and it really hot. So whenever I see Kim Possible, I think of my hot friend. All in the association.