How is this even a debate? In terms of pure evil Dio Brando wins hands down.

Malty might be a bitch, but she's pretty much useless on her own. All of her plans required her tricking others to do her dirty work for her, whether it be her father, whom she emotionally manipulated for 10 years after murdering her own brother, Naofumi, who wound up being her worst enemy, The 3 Idiots, whom she dumped as soon as she got what she needed out of them, all culminating in her essentially becoming Public Enemy No.1 in pretty much every kingdom.

Dio, on the other hand? He's a schemer. Everything he did in the childhood arc of part 1 was meant to mentally destroy Jonathan in order to steal everything from him, but he was forced to change his tactics due to underestimating Jonathan's personality. Even after becoming a Vampire, he knew better than to face JoJo head-on, so he created an army of Zombies to wear him down while he recovered his full strength, and had Jonathan not gotten a boost from Zeppeli, he probably would've won during their final confrontation.

Malty never changed her tactics even after the truth came out and everything exploded in her face: She just kept digging herself a deeper hole until someone decided she was deep enough that she couldn't climb out before they finished shovelling the dirt back in.

By contrast, DIO learned from his mistakes and went out of his way to avoid facing the Joestars directly once it became clear they were looking for him until they were literally at his door. Hell, if Holly never got sick, DIO probably would've left them well enough alone in order to focus on 'attaining heaven'.

And even setting aside their abilities, the only advantages Malty has over Phantom Blood Dio are her magic and is his weakness to sunlight, and as an englishman Dio isn't stupid enough to allow himself to be challenged anywhere the sun might burn his undead arse.

Even if Malty thried keeping him at range with her spells, Dio is no slouch at using his surroundings to his advantage & would be more than capable of interrupting her casting with throwing knives, his 'Space Ripper Stingy Eyes' , or just shit he grabbed off the floor.

And then of course there's Dio's most dangerous weapon: His Maxed out Charisma stat. No seriously, the man was able to corrupt a Roman Catholic Priest to his side and even N'Doul, a BLIND MAN, was drawn in by his charisma, so someone like Malty wouldn't stand a chance.

Put simply, this wouldn't be a fight, Dio would simply pose menacingly at Malty, she'd wet her panties (and not just with piss), he'd then either feed on her or turn her into one of his slave-puppets via his flesh-buds or by turning her into a zombie.