Chavez Sr. fought on longer past his prime than Hagler. (So the pop vote will go to Marvin) That said... Hagler one of the best of all-time (Chins)
Both were as close to undentable as it gets in their prime, and proven against fighters with genuine knockout power. JCC obviously stayed on long past his prime, so there are images of him being dropped and stopped that never happened to Hagler (save for the dubious call against Roldan). Overall though, Hagler proved his chin against better punchers than Chavez, and showed hardly any vulnerability in more than a decade at world class level.