Is Rb F a ionic or covalent bond?

Yes it is an ionic compound. Rubidium(Rb) is the cation with 1+ charge and Fluorine(F) is the anion with 1- charge. They are formed by the donation of one electron from the valence shell of Rubidium to Fluorine and hence they form an ionic bond, which is the electrostatic force of attration between two oppositely charged particles.