During One Piece's "Wano Country Arc," everyone's favorite cook went through a surprising transformation. While fighting Queen, one of Kaido's Lead Performers, Sanji awakened the Germa 66 powers everyone believed he was born without. While it gave him a hefty power upgrade, Sanji himself seems particularly unhappy with the development.
The powers Sanji awakened came from his father, Vinsmoke Judge -- more accurately, they were a result of his experiments. While Sanji was in his mother's womb with his three brothers, Judge performed experiments on the developing fetuses in order to turn them into merciless superhuman soldiers. With super strength and durability, and without the ability to feel sympathy or empathy, Judge's experiments were almost considered a complete success.
Sanji's Weakness Made Him a Target for Abuse
They would have been too, if it weren't for Sanji and his mother, Sora. She took a drug while pregnant in an effort to stop what she believed was a cruel experiment, specifically the aspect of it that would remove her children's emotions. While she was largely unsuccessful, she did manage to save Sanji from that fate; however, it also prevented Sanji from receiving all the physical enhancements. Born as a normal child, it didn't take long for his siblings and father to target and abuse him, especially after Sora's death -- a side effect of the drug she took.
While Sanji was considered powerless, his siblings were the complete opposite. Gifted with super-powerful exoskeletons and healing factors, each of their bodies were practically indestructible. They were also granted incredible strength to go along with their durability, allowing them to shatter stone at an impossibly young age. Beyond the standard enhancements his children received, they each were granted specific abilities completely unique to them. While it would be easy to believe that these powers came from the Raid Suits, in reality they come from their enhancements. The Raid Suits instead provide other kinds of abilities, such as even higher physical stat boosts and further enhancing of the unique powers the children possess.
Why Sanji Is the Strongest Vinsmoke
Reiju, Sanji's eldest sibling and only sister, gained poisoning abilities as well as the ability to cure poisons. Ichiji gained the power to create energy beams and explosions, Niji gained electric powers and Yonji gained even greater physical enhancements than the rest of his siblings. Paired with their special Raid Suits that only work for them, each of the Vinsmoke siblings has the potential to be stronger than an army. Together with their father, they form the evil team Germa 66.
With so much power between them, it would be easy to assume that they are unbeatable, but Sanji himself managed to prove that untrue. With only his training and Haki, Sanji managed to completely overpower his brother Yonji, as well as force his father to cheat in order to win their fight. This was all before any of Sanji's Germa 66 powers activated, proving that the experiments weren't perfect.
Sanji's powers seemed so much stronger than his siblings'. Rather than always having their monstrous strength, Sanji had to train to get his. This made his power-up much greater, as his super-strong adult body was the foundation, instead of a child's. As strong as the Vinsmoke children are, it would be hard to imagine that they could have tanked Queen's sword to the neck the way Sanji did. With a combination of his body's own strength and enhancements, Sanji is easily the strongest Vinsmoke.
Sanji Destroyed His Raid Suit to Sever All Ties to His Family
For a while, Sanji's true power was a mystery. While each of his siblings were given special powers unique to them, Sanji's unique power was unknown. Queen implied that his iconic Diable Jambe was somehow related to his enhancements, but since Queen was just guessing, it can't be said for certain. It could also be related in some way to his Raid Suit's ability to turn completely invisible. While it seems that was an ability of the suit's and not Sanji, it could be a bit more complicated. If the Raid Suit was what activated Sanji's enhanced genetics, it's entirely possible it was his own power all along, and he didn't realize it while using the suit.
Regardless of how much stronger Sanji has become, he is terrified of getting even stronger. With his kindness and compassion being his greatest qualities and arguably his most defining traits, Sanji's worst nightmare would be becoming like his siblings. Spending most of his life trying to be their exact opposite and live the life he knows his mother would have wanted him to have, Sanji is rightfully terrified of these changes. As Sanji began to understand the powers rising to the surface while wearing his Raid Suit, he destroyed it in an effort to sever all ties to his family and the Germa Kingdom.