Vega's a sadistic and psychotic murderer, but with enough sanity and chivalry to not make him a total monster.
Balrog isn't really super evil, is selfish and not stranger to killing, surely far from being a saint, but not even a devil.
Bison on the other hand has a full collection of crimes and sins including genocide, murder, mass destruction, running the world's underworld, weapons and drug trade, money laundering, bribery, warlordism, being a traitor and a literal dictator, human experiments, child abuse, brainwashing, controls a power that literally stems from cruelty and chaos and feeds off fear, despair and hate. He has also tried to replace the will of every single human being with his own and the list goes on and on and on...
For all the widespread variety of evil deeds that Bison has committed willingly and voluntarily, I'd dare to say he is indeed more evil than the various Satsui no Hado madmen, as they did indeed embody carefree and meaningless destruction and murderous intent, but they are more akin to insane and uncontrollable forces of nature.
Of the many evildoers in the SF verse, I believe none reaches the same level of pure, hopeless cruelty and intentional disdain for everything that is good and for human life in general as Bison.
