So many villains to choose from...
Well, let's start with the guy I had as an avatar, sig and personal photo before: Rugal Bernstein!
He's like, this insane guy who is richer than Bill Gates (probably) and has mastered every martial art in the world which is made publicly available. So he tries to become even more powerful, and seeks to steal the power of Orochi (an ancient being of great power). So one of Orochi's 8 Ketsu comes and gives him a beating to teach him not to mess with the Orochi, slicing out his right eye in the process. But Rugal somehow survives his attack (which shows how powerful he already was). Impressed, Goenitz (the guy who beat him up) gives him a portion of the Orochi power, which makes him fall unconscious. After a while, he wakes up and realizes he has succeeded in getting what he wanted. He then gets a bionic eye to replace his missing right eye, and organizes a tournament to crush the best fighters in the world with his new found strength.
He has much more to him than that, but it's kinda hard to explain.