Which Villafranca? There are many!

If you mean Villafranca del Castillo, which lies some 25 km to the west of Madrid, off the A-6, buses 627 and 623 go there from the Moncloa area of Madrid. See http://www.autoperiferia.com/presen.htm for schedules. There is no bus from the airport that goes to Villafranca del Castillo. I would take a taxi (and have done so - you drive around the city instead of cutting through, so it goes quite quickly).

If you mean a different Villafranca, let us know!