He survived 75 days without food as a child. Most humans can barely go 2 weeks before dying.
He sacrifices himself for the good of others constantly, thinking of them before himself.
He's Oda's go-to when he criticizes bad royals vs the values they should have.
As the thread creator said, he constantly turns enemies into allies, and brings out that small amount of good in bad people.
He's empathic, feeling others pain and understanding them rather than just pitying them.
He's shown leadership qualities when Luffy is still there even, in both Skypiea and Enies Lobby when he took charge and the others listened to him.
He's stubborn to a fault about saving people, the only time this came in question was the kids in PH but he was conditioned to believe from his own childhood that people don't help 'experiments' even if they're children. Aside from that he consistently saves those he even said he wouldn't save and is used as a clutch.
What did Killer do? Follow a psycho murderer like Kidd who killed innocents for fun or because they were in the way, the ones Oda has written the story against so many times (clearly the reason he had Kidd fail and Law survive.)
Sanji is a hero, and a true leader. That's why. As they say, while a King is a leader, they're at the same time the SERVANT to the people. And Sanji is definitely that.