To me there is no one more wicked in nature and deed than Wapol on the list. He seems to come out last on everybody's list while I have him as a clear number one for many reasons.
He is by far the most ruthless dictatorial piece of shit of the lot. To cement his utter and absolute control over the denizens of Drum Kingdom he kidnapped all the doctors it had and perpetrated a Stalin-like great purge on those doctors who resisted; these were the most renowned doctors in the world who he had no qualms about killing to further his power grab, proving he holds to no higher purpose or goal like progress or bettering human welfare: he only looks out for number one and how to best exploit other people. To further illustrate how no one but himself exists on this guy’s radar, he was ready to instigate a fullblown war with the Alabasta Kingdom simply to balm his wounded and childish pride for having been criticised at the World Summit by Cobra over a total triviality. He doesn’t give any hoots in the slightest if his country withers and everybody in it dies, as long as he reaps all the spoils. The guy holds to nothing and no one other than himself and his own self-aggrandisement.
At the very least BM wants to create welfare for her citizens, and Orochi seems to shower boons and parties for his allies and seems to take great pride in being a self-sufficient country that can provide for its citizens (though in the capitol that is, though be it at the expense of the outside regions like Kuri). I can’t imagine Orochi doing what Wapol did and abandon the countries defences and take all doctors with him, leaving his people in utter desolation. Walpol harbours no shred of ideals or national pride, and certainly no allegiance to anyone but himself. His vision and end goal is in fact to create a situation where his citizens are slaves living in poverty and in a state of total dependence to him to ensure total control over their lives. Wapol was sheer and cold evil through and through.
Unlike Doffy, Crocodile or even Kaido, they either have the ability to foster relationships (Doflamingo held genuine love for his family) or have good end-goal intentions that I’m sure you can feel sympathetic towards, if Crocodile and Kaido had a chance to lay out their vidions and explain why they had to do what they did. The goals never justify the means, but at least I’m willing to entertain the idea that they had much grander visions that extended beyond just their personal self interest, whereas Wapol only cares about himself and seems perfectly capable of their level of atrocities provided he has the means to do so and if the outcome thereof ensures he comes out on top.
I think Enel is close to Wapol, but he never went out of his way to directly enslave the populace by stripping them of life essentials the way Wapol did. Enel was so caught up in his God-complex delusion that I don’t think you can even assign him the same level of agency. He is simply not of sound mind, non compis mentis. In that sense I regard Wapol as being more evil, capable of the same atrocities had he had the same means to do so, only he is not delusional, but purely calculating and conscious about the evil he does, taking great satisfaction in perpetrating it