Who's the worst president the US has ever had?

Plenty of choice. In my view, most presidents were rambling reeking right wingers in some way or other, save for FDR and Teddy Roosevelt, who were the two presidents I'd actually call capable and outspoken on civil rights (rather than just pragmatical like Lincoln). They did have their blemishes, but less than e.g. Andrew Jackson.

So many presidents were terrible for one people or another.

Andrew Jackson? Held hundreds of slaves and quite literally led an ethnic expulsion against Native Americans (the Trail of Tears).

Lincoln? Mostly good, but did not forbid slavery in the form of penal labour. If one were to abolish slavery, why not go the full mile?

Wilson? Rabid antisemite, pretty much.

Hoover? Might've tried to tackle the Great Depression -- but did so by allying with large coorporations, effectively being corrupt and choosing bribery.

Truman? Dropped nukes and set the stage for "we support any government that hates people being remotely leftist".

Nixon - corrupt and wanted to sidestep the rule of law, all for his own profit: to stay in power. Other than thaf, decent, but that's a big "other than that".

Reagan - enough said. Ultracapitalist, misleading, made the US economy far worse by accruing debt like there's no tomorrow, and shoving it onto the poor -- typical oligarch behaviour! Militaristic, power-hungry. And no, he did not end the Cold War: Gorbachov did.

JFK: socially pretty good, actually. But economically, the cutting of the top rates made the richest keep more money. At least it wasn't down below 50%, but still. Had that happened, I think the tax rates would've allowed wealth accumulation.

And so on.

So, in my view, it's hard to focus on who is the worse, and better to rather focus on what is the best. Ted would be my candidate. Not only social progress, but also economical, and in a way that favour the worker -- and he also was environmentally aware. That is a good president.