The correct answer is $\boxed{\text{CsF}}$.
Lattice energy is the energy required to separate one mole of an ionic solid into its constituent ions in the gas phase. It is a measure of the strength of the ionic bond. The lattice energy of an ionic compound is inversely proportional to the radius of the ions. This is because the smaller the ions, the closer they can get together, and the stronger the ionic bond.
CsF has the largest cation and the smallest anion of the four choices, so it has the highest lattice energy.
KF has a smaller cation and a larger anion than CsF, so it has a lower lattice energy.
NaF has a smaller cation and a larger anion than KF, so it has a lower lattice energy.
RbF has a smaller cation and a larger anion than NaF, so it has a lower lattice energy.