Disagree about Scarpia, raping women goes well beyond "doing his job"....and the statuette goes to ... IAGO!
First of all, Pinkerton is not inherently evil. He's an a**hole and unthinking but he has a change of heart and even excoriates himself for his past selfish behavior, meaning he has a conscience.
Scarpia is just doing his job but he's not thoroughly evil. He's egotistical and full of himself and enjoys exerting his power position. He even gives his vulnerability away when he says, "Tosca, you make me forget God!"
Claggart (who probably comes closest to Iago) has an axe to grind being that he has found himself in the untenable position of having romantic impulses toward Billy Budd and having to squelch them. He is mean spirited and psychologically insecure but that doesn't necessarily spell evil.
Don Giovanini is too busy being a Narcissist to be evil incarnate. Even he himself was shocked that he went as far as to kill the Commendatore. He actually didn't mean to do it.
Now Iago, on the other hand, is the epitome of evil because he has no conscience whatsoever. He admits it in the Credo.
He couldn't care less about anyone or anything (including his own wife). He's doubly dangerous because he has a certain charm that wins over people (typical of psychopathic behavior)
Oh come now, Pinkerton is a nice young man who acts without really considering the consequences of what he is doing. That young men can and do such things in their youth only to regret them later is a sad fact of life. His remorse is genuine. We don't know how he lives with himself later on. The opera ends with him finding the body of Butterfly. That is probably a pretty hard thing to live with for the rest of your life.Like the title says, who is Opera's greatest villain and why? Sure there is Hagen but he was only carrying out the family honour, where Pinkerton is a fiend!