I don't know much about the character, but I just looked him up and he seems pretty evil. He seems pretty close in nature to Muraki Kazutaka (who I think is the best anime villain ever and, ironically, voiced by the same guy in Japanese), so he's probably one evil dude. However, Muraki's rather watered down in the anime, so it's possible that Aizen could be the best anime villain, but Muraki is one freaky psycho in the manga, so he wins 100% there. But I don't know for sure. I've never seen Bleach.
But Muraki wins because of the psychological games he plays and the way he over does everything. For example, his old mentor, Satomi, is doing a cloning experiment that requires human hair. But Muraki doesn't go around asking for hair or sneaking up on people and grabbing some. Nope. He starts killing women just for a tiny sample of their hair. And then, 3-5 years before the start of the story, Muraki kills a woman and Hisoka sees. Now, Muraki has the power to erase memories, so he could've just wiped Hisoka's mind. But no. He rapes him, marks him, and then curses him to die slowly and painfully over the next three years. Oh, and erases his memories. ^^ And then there's Muraki's main goal... Basically, he wants to chop of the head of someone he "loves" and stick his dead brother's head on the body, bringing him back to life just so he can kill him. Muraki's so twisted I love him.