Who ,in your opinion , was or is the worst American president in history and why ?

Please , Bush-haters , try not to get too vocal on this one and please do try to stay within the parameters of the question ( ie. don't go ranting on about " Imperialist aggression " etc.) as I don't want this descending into a "We Hate Bush " rally.
Just stay concise and pleasant and I'm sure everyone'll be happy.