Orochi = used massive amounts of cow fodder to attempt to achieve his goal, which is kinda why he is viewed as evil. granted plenty of his experiments help the current ninja world in some indirect way. however he deff has charisma and uses it to a great advantage (kabuto, sasuke, sunakagure, the entire sound village etc.)
Danzou = never really considered a villain in my eyes because if he truly was hiruzen prob would have kicked/killed him a long time ago.
gaara(pre-shrink) = just wanted to kill every1 to satisfy his "mom" and underdeveloped bloodlust.
Pein = you will know suffering... i has haxor eyes and can control life n death soooo...

good luck.
lets not get confused with opponents now, opponents does not necessarily mean villain.
most of akasuki past and present would probably considered opponents rather than villains, save pein, tobi, and maybe even zetsu.
Sasuke -> orochimaru: Oro y u no spam good move?
Oro -> Sasuke: maybe because I want to know the all... that's why you're here.
Sasuke -> Oro: I see we have come to a disagreement... I have nothing left to learn from you. You taught me a awesome move that I will spam to greatness, F other jutsu (dam I guess I am turning into naruto)