Well when you post that picture I want to say Angelo.

A reminder Angelo was a rapist at age 12 and is a serial pedo/child murderer. And he would do wayyyy worse if the resources of DIO

Dio

People always say Dio, and tbf Dio is comically evil. He's a psychopath, and borderline sadist. But he's not a pedophile. (You could argue he's a rapist maybe). It's not necessarily the most realistic, or even terrifying depiction of evil. His evil is charismatic, flamboyant, and cartoonish, akin to Frieza (who genocided entire planets, yet still is treated thematically in a way where that sort of gravitas doesn't actually effect his character). What I mean by this is, there's some suspension of disbelief with Dio. With Angelo, his crimes are real in their parallels with real world criminals. He's not some alluring vampire who makes people eat their babies for fun. And in that way, the story and the audience takes Angelo much more seriously then Dio. You could even argue that Dio is simply detached from humanity overall. It's shown in his very transformation into a Vampire, when he says "I reject my humanity!", meaning it's not even, necessarily, a situation where he commits evil acts out of any real desire, it's often just amorality. He sees killing as eating bread after all. Dio's evil is more akin to a human torturing an ant, incapable of empathizing with the ant due to the huge gap between sentience. Dio's main goal isn't the continued deaths of babies everywhere, or to dump dogs in furnaces for any se3ual gratification. It's to ascend into heaven, and follow the ultimate defining characteristic of Dio, his desire to always be on top. If anything the only real evil of Dio is his pride, and how it supersedes his ability to even think or act as a human, even when he was younger before becoming a vampire.

For Angelo there is no 'greater purpose'. No ends to justify the means, no inherent detachment. He is human at it's cruelest. He acts upon his desires and impulses, and commits heinous acts that, in my opinion, are worse then Dio, and he doesn't do so with the detachment of 'crushing an ant', or 'eating bread'. Angelo is less alien, more recognizable with real figures of 'evil' status in our world, and I feel fits that label more appropiately because of it.

Angelo in my opinion just represents a far worse, real world, and intense evil that's taken far more seriously, and his character reflects that for me. So for me personally, Angelo is the most evil character in the story.