What is the conjugate base of HSO_4^-?

The conjugate base of any species is the original beast LESS a proton, ${H}^{+}$.
$H S {O}_{4}^{-} \rightarrow {H}^{+} + S {O}_{4}^{2 -}$
Bisulfate is still a fairly strong acid; as the conjugate base of sulfuric acid, ${H}_{2} S {O}_{4}$.