Hiding beneath plenty of valid answers is the discussion of what constitutes evil. We tend to define it by societal standards of the time, but it gets dicier when you can have characters who don't realize what they do is evil (mindless), those who believe what they do is righteous (Doom), those who recognize their actions as evil but don't grasp the entirety of them or revel in them (Carnage), which means the weight of their actions never fully sinks in. And the last category (and probably the most truly evil), those who understand their actions on a deep level and still choose to commit atrocities.
Maybe Joker. He's a tonal cousin of Carnage, so he might get a pass from being mad, but I think he knows what love and kindness is. He knows that he's the antithesis of them. His madness doesn't blind him from understanding morality, and happiness vs. pain and suffering.
I think that evil and maniatical suicidal people are, yes, quite evil, but for me it's even more offensive to be all that but also a coward and power hungry bastard that wants to control everything (which is something Joker, Carnage or Bates honestly lack). Arguably Kid Miracle Man pretended to be a businessman for years and had the potential to be that kind of villain, but his vicious nature very much overshadows his ability to rule through fear an slander.
So if I was to chose someone as the top evil I'd probably go with something more akin to Lex Luthor, but he's not nearly powerful or succesful enough (damn you Superman!), to qualify as the bigger evil. He has the pathetic pettiness and the psychical cruelty to fit the bill though. Instead I'll go with someone that was also quite ineffective in several ways, but whose pathological evilness is arguably on a bigger scope than everyone else discussed here: the god from Preacher. I think the character is molded over the idea of how a dictatorship would work, kind of the physical representation of a Cult of Personality. Which by itself is a pretty nauseaus concept.
In case this link gets destroyed, it's a short clip of Owlman describing his TRUE plan for the "Q.E.D." bomb in the "Justice League: Crisis on 2 Earths" movie (to Superwoman of the Crime Syndicate). Strangely appropriate: The Owl or "Owlman" is an historical symbol of a god of death called "Molech" or "Moloch".
Doubtless this is why the League or Court of Owls, though having no association with THIS version of Owlman, is a secret club of assassins.
Owlman, of course, is the Crime Syndicate's version of Batman.