Goku and the other Saiyans were not born powerful. Goku even after becoming the strongest on Earth was still nothing compared to Raditz, who was nothing compared to Vegeta. The only reason Goku is as strong as he is is because of the fans boys and because of all his training. Without that he'd be a below average Saiyan. You can't use the "He's a God, he's the strongest." argument either because any pure blooded Saiyan can become a God in the right situation.
Broly is different because he didn't have to work for his power like the other Saiyans. He was born powerful with power level of 10,000 which was more than King Vegeta,Prince Vegeta, Nappa, Bardock and all the other saiyans. he got very little training because he was restrained by the ring around his head. He also has unlimited energy in his LSSJ form, making his power limitless. He can become as strong as he needs to be just by being Broly. Any Saiyan can be a God, but only Broly can be the Legendary Super Saiyan. Everyone else has to train and train just to survive but Broly's awesome power comes naturally, making him superior.
Broly is the strongest Saiyan. he's the most powerful in the sense that his power comes naturally rather than being worked for. He doesn't have to devote his life to combat like Goku or work out in a gravity chamber like Vegeta. He was just born with it .