It's really quite difficult to rank one as 'the worst' because they all existed in very different times and left very different legacies. That "Trickle Down Economics" farce is reason enough to rate him pretty high given the legacy it has left. However, I think Clinton's deregulating of the banks did far more damage. Then you have Bush Jr. and the invasion which is not very popular anywhere and was arguably a mistake given what rose from the ashes of Saddam's Iraq. You've also Truman who's the only sitting leader to have ever utilised a nuclear weapon... justified or not, that action cemented the U.S. as a warmongering nation that will do 'whatever it takes' to achieve its goals. As for Obama, he's presided in one of the most polarised times in U.S. history, and as a 'leader' has absolutely failed to to foster a desperately needed bipartisanship. ....and those are only some of the more recent ones...