Just to toss out some redeeming characteristics for some of the usual assumed crappy presidents before I get to my pick for the worst:
Harding: He was a fuckup in the sense he had a shitty judgment of character when it came to his subordinates. His actual record aside from his terrible judgment in friends is actually decent if not outstanding, given he did a good job trying to fix the domestic economic pit Wilson left the US in due to WWI. He was one of the few pre-Great Depression Presidents to try and make the farming sector not suck so much, and while he didn't live long enough to do much about it, he denounced racism pretty hard.
Ford: Pardoning Nixon was a good move, Nixon's acceptance confirmed he was guilty as shit without dragging on hearings for years. His attempts to fix the economy weren't all that great, but at least he tried.
Carter: His foreign policy was highminded, but honestly, imploded on itself most of the time. Trying deal with countries on a moral basis when that just makes them think you are weak backfired hard, and Castro trolled him hard by dumping his lunatics and criminals on us. Have to give him credit for at least coming much farther than anyone else to getting peace for Israel implemented, not exactly his fault everyone else involved didn't want to play ball.
Wilson: He was a massive bigot, an authoritarian douchecanoe, and the League of Nations bullshit was the most autistic shit ever. On the positive side of the ledger, at least during his first term he did shore up the domestic front pretty decently, and he was fairly competent in his handling of World War I, he just sucked absolute ass in the aftermath.
@Save the Loli - Zimmerman telegram was actually real, but it's importance was pretty overstated, Mexico told the Germans to shove it, but Wilson used it as part of his excuse along with the resumption of unrestricted submarine warfare to get Congress to declare war. I consider WWI a net positive economically for the U.S, it completely negated any credit deficit with Europe for quite some time.
Nixon: When you remove Watergate from the equation, he did a decent job trying to keep the economy from falling in on itself for awhile and his foreign policy based on realpolitik was pretty effective.
Grant: Yes, he proved far less competent than everyone thought, even if you liked him as a human being. On the good side, he fought tooth and nail to keep the era of Jim Crow at bay while he was in office. And while a lot of corrupt shit happened during his presidency, he was never personally involved, he just had Harding's problem of judging friends poorly and some crap, like the Credit Mobilier affair, had nothing to do with him, he got the blame anyway.
L.B. Johnson: Vietnam was a shitshow, no question. The Great Society program was certainly overambitious and the welfare state he helped set up was dumbass in hindsight, though some good laws like road safety regulations were born of that period at least.
Now that I covered them, I'd say both Andrew Johnson and James Buchanan share the "worst president crown".
In Buchanan's defense, he was handed an absolutely terrible situation, but he proceeded to handle it in the most stupid manner possible, to the point his last few months in office he proved so inept he was practically helping the proto-Confederancy.
Andrew Johnson is his post-war counterpart, promptly running any attempt at fixing shit into the ground just so he could slake his own ego, to the point that jackass set about inflaming tensions Lincoln knew he'd have to settle down after the war if he wanted the South to play ball again, which resulted in the absolute shitshow Reconstruction was.
