Okay, I have been watching the show since the mid-80's and I would have to say that while I enjoy Victor the most, a lot of his evil has been to protect or correct his family's follies. In regard to all out evil, there is no one more wretched than Stefano.
Yes, in recent years, they have humanized him way too much, in my opinion. But, this man has returned from the dead more often than Joan Rivers has had her face stretched. His persona, the Phoenix, just goes to show how evil and cunning this man is. None of his heirs, including the illustrious E.J., can hold a candle to Papa D's whacky ways.
Sami is about as intimidating as a blind rabbit. Half of her evil deeds were merely done to catch this man or that man. That doesn't make her evil. She's never intentionally killed someone, kidnapped someone and brought them back as another... etc.
So, for my nearly 30 years of watching, I'd say the title of worse Days villian belongs to Stefano DiMera. Vile as all the others may be, he's the only one whose possessed the kind of staying power to hold this title. Just my humble opinion, of course...