Most disliked - no contest, Bush Sr. He's the very reason why there should be no politics and no politicians, because we can't avoid people like him. He is cruel, cold-blooded snake, ready to sell his own mother for some sociopathic self-interest. I am not completely convinced that any president after him could operate without his long hand interfering in some way.
Nixon - complete bastard, but at least he was a little more open about it than Bush Sr. They are both very much alike, only Bush Sr. is the more introverted version of the same condition /which makes him worse/.
Washington - he founded a country, made of slave owners, who wanted to be called "free", and after they've almost completely destroyed the natives. He said people don't deserve freedom and security if they don't want to fight and defend such rights. This counts as quite the cruelty in my book. Hopefully it will soon remain as one of the horrors of the past.
Ron Paul too, if he gets elected. I wouldn't trust this guy to care about his own mom, let alone a nation, or humanity.