Lisa, BB7. She dripped poison in Sam's ears about Lea, and then told Lea that Sam was bitching about her. Lea confronted Sam, who told Lea about Lisa starting it, and when questioned (and seeing how upset Sam was), Lisa denied it. She's remembered for her annoying loudness and swearing, but this was her worst moment for me.
I'm not saying she's the nastiest housemate ever, just adding one not yet mentioned.
Also, Charlie, BB10. He was downright cruel sometimes, and defended his actions as "only having a laugh". Like when he soaked Angel with a water balloon, reducing her to tears. He also got involved in the Siavash girlfriend controversy when it had NOTHING to do with him, and I remember, when they read magazine articles, he read a scathing comment from Bex (BB9), and called her something along the lines of a "silly fat bitch". I didn't like her, but weight jibes piss me off. He also admitted that he assumed that the public would see him as the most deserving housemate to win because of his mother's story.