Which combination would give the fastest conversion of 1-bromobutane into 1-fluorobutane using potassium fluoride?

$\mathrm{F}{}^{-}$ is heavily hydrogen bonded in all the protic solvents (water, methanol, acetic acid), which strongly diminishes its nucleophilicity, and KF is poorly soluble in aprotic solvents. The crown ether sequesters $\mathrm{K}{}^{+}$ in acetonitrile, solubilising KF and leaving an unsolvated, highly reactive fluoride.