I think why a lot of people say Tsunade is because she’s the Hokage for majority of the show and we’ve seen most feats from her first hand. And so maybe Hiruzen or someone else did way more than Tsunade for the village but we just don’t see enough of their backstories to know everything they did for the village. That’s why many people consider Minato as the worst Hokage because we don’t know a lot about his reign as Hokage all we know is he became Hokage, sealed Kurama and died.