Well if I had to pick my #1 it would probably be Pol Pot, he killed more people than Hitler did, and didn't even seem to really have a viable plan or even think one was nessicary. He also did so in intentionally cruel and torturous ways. #2 would probably be Charles Manson because his manipulative goal was pretty much expressed as to committ evil and chaos as I understand things. It's a rare person who actually wakes up in the morning and goes "gee, I feel like being really evil today".
Hitler and a few other popular choices would be fairly low down on the list for various reasons. Since he's obvious I don't rate him higher because I don't think the number of people he killed is really as relevent as his motives, and honestly he at least meant well and believed what he was doing was for the greater good. What's more while people hate to admit it, the man was a visionary and did a lot of good, having become an international "man of the year". This is incidently one of the things that makes him so frightening, and one of the reasons why I frequently chime in when people compare an unpopular leader especally in wartime to Hitler. To compare to Hitler you really need to find someone with global populaity who is right about 95% or more of what he says and does, but is totally flipping insane within the other 5% and yet manages to pass it off to the point of converting tons of fanatical followers. People simply do not seem to learn the lesson inherant here.
Honestly I don't spent enough time contemplating psychopaths and their motives to have a really fair "order of evil" by my standards, but for me it has a lot to do with intent. This doesn't make someone with good intentions right or justified (we all know what the road to hell is allegedly paved with) but I think motives matter when talking about good or evil (and it can get very complicated from where I sit). There aren't many people who actually did things because they embraced evil for the sake of evil, but they are on the top of the list. Pol Pot is the worst I can think of because he went beyond that and claimed to have reasons but really in the end I think he was closer to what Hitler is accused of than Hitler was. He pretty much just wanted to kill a lot of people and seemed to work his justifications around it to rally people. The thing about Hitler was he didn't just say "I want to kill millions of Jews and Gypsies because they are differant and I don't like them", he was very charismatic and him and his men could point fingers to the art treasures and such being recovered from the basements of Synagogues, hidden fortunes, slum lords, real estate scams, organized crime, and other things in many cases to justify his actions and even present evidence. That doesn't mean we was right and wasn't overreacting to an insane degree and holding truely massive numbers of innocents accountable for things they had nothing to do with, nor do I believe he had exhausted all other possibilities, including some rather extreme ones that would have been nasty but were not genocide. Basically he was wrong, and he WAS evil in the final equasion, but he wasn't as evil as many other people. I do not think he started out with the desire to kill millions and then worked on finding ways to justify what he was doing aftrwards for example.