I personally think that the rudest judge on television is that judge from America's Got Talent. Everything he says is rude and not even close to being constructive. It's like he's trying to be a character.
This guy! Piers Morgan.
I hate him!! He was a judge on Britain's Got Talent alongside Simon Cowell and everytime Piers would say something nasty (when he clearly didn't have a clue what he was talking about) the audience would shout, "Who are you? Who are you? Who are you?"

He tries to be somebody he isn't (even though both personas come across as complete a-holes) and nobody really likes him.
But out of the judges listed I would have to go with Janice too. I don't watch America's Next Top Model but have seen like one episode and she's just horrible to try and be funny, whereas when Simon does it he's hilarious anyway and actually tells the truth and knows what he's talking about. He also knows how far to push it too.