Now, let me clarify what I meant by that. Who could kick serious butt? Who has powers so incredible that anyone who took him/her on would be toast unless they had a lot of skill and a lot of luck? (but if you want to say who you think the most evil villain is, that's fine by me).
Now, Suzaku (the fourth Saint Beast from "YuYuHakusho") had a technique called prism of seven. Being able to attack from multiple angles is a great power to have. And each Suzaku was an individual entity that could act independently of the other... even do a new prism of seven if the others were destroyed (although he could not go above seven at any given time...). So, the only way you can defeat him is if you destroy all seven at once!
This sounds a lot like Naruto's signature technique, Kage Bushin no Jutsu (Art of Shadow Replication) the difference being that you don't have to destroy all the clones at once-but he makes up for this by being able to produce hundreds of clones instead of just seven.
Anyway, I always liked Captain Kuro from One Piece for sheer nastiness. Not only was he truly cruel and heartless, he's one of the few villains who can challenge Luffy WITHOUT relying on a Devil's Fruit power-his ability to run so fast you can't see him (while eviserating everyone in his path with his Freddy Kreugeresque claws-even his own men) and his fighting style is enterily self-taught. But the best thing about Kuro is that he's really, really smart-Oda-sensei says he's the second smartest person in all of One Piece (the first being Ben Beckman, Shanks' first mate-the tall guy with the split curl who stands next to Shanks in the second opening) which earned him the name "Kuro of 1000 Calculations."Intellegence, when put to evil use, is very scary-he successfully faked his own death, conned an entire town, and he would have gotten away with it, too, if it hadn't been for some meddling rubber kid in a Huck Finn hat.
Also: Master Asia from G Gundam. Not only is he like a father to the hero, he's insanly powerful, driven, and worst of all: politically connected. Being unofficial advisor to the man who controls the world makes one pretty formidable. And the final battle between Master Asia and Domon is one of the best anime fights ever.