Which weak acid has the strongest conjugate base?

Acetic acid $\left(K_q=1.8\times {10}^{-5}\right)$

Hydrofluoric acid $\left(K_a=6.8\times {10}^{-4}\right)$

Propanoic acid $\left(K_a=5.5\times {10}^{-5}\right)$

Formic acid $\left(K_a=1.8\times {10}^{-4}\right)$