What made Hiruzen stronger was his raw, genius ability at all the basic fundamentals of what a ninja needed to be. He made up for what he lacked in genetics with insane skill and proficiency at pretty much all things. It's how he became known as the "professor"
He was also a master of close quarter's combat. Anybody who can slip two explosive tags on the legs of two Hokages without them realizing, while also engaging Orochimaru in a close range weapons battle while Orochimaru is wielding his best weapon, and then successfully disarm and completely distract Orochimaru, while at the same moving in quickly from his blindspot to grab and hold onto him and use a technique on him at close range, is incredible!!! He's basically a perfected Kakashi with Shikamaru like thinking on steroids with immense chakra reserves that made the whole building him and Orochimaru are fighting on crack.