Most sympathetic would probable be one with a tragic backstory who while in a sense "evil" is not evil evil if that makes any sense. They try to avoid it a much as possible. The only one that springs to mind at the moment is Magneto, but I'm sure there are others. Doom doesn't fit because too often he crosses over into the evil category.

Least sympathetic would have to be those who do it for completly selfish reasons or for evil's sake without any justifiable reason at all. Thanos has done some pretty bad things. So has Doom. Purple Man has used his powers to pretty much rape women and force others to do bad things while enjoying it. That right there make him one of the worst. Loki has made Thor's life miserable. Let's not forget Mephisto who aside from embodying evil had made pretty much every hero's life miserable at some point.