I went with Angelus. He actually knew what evil was and became an artist with it, and he truly wanted to destroy and harm as opposed to dominate. (Warren was a wannabe.)
Caleb came very close because he, like Angelus, wanted to end the world in misery (something none of the others did, though Glory didn't care if that's what happened), but Angelus exulted in the suffering so much more. I doubt Caleb could get anyone to immolate themselves while delivering a message (my mind marvels at how Angelus accomplished that), though I'm sure Caleb would if he could figure out how. And when Caleb smirks or smiles, he comes off as bit of a kid to me (what many would call a punk in the pathetic loser criminal sense--in that sense, a lot like Warren actually), but when Angelus smiles (at least in Buffy) I shudder. Even when Angelus is getting the crap beaten out of him he seems to enjoy it, because he's exulting in the pain he inflicted to get such a reaction.