Answer
$HP{O_4}^{2-}$ is the stronger base.
Work Step by Step
The weaker the acid, the stronger its conjugate base. Therefore, if we determine which conjugate acid is weaker, we will get the stronger base:
Conjugate acids: $H_2P{O_4}^-$ and $H_3PO_4$;
Since it is easier to take a proton from a neutral compound, $H_3PO_4$ is the stronger acid, and $H_2P{O_4}^-$ is the weaker, which makes $HP{O_4}^{2-}$ the stronger base.