It's tempting to pick someone recent, but the real answer is probably Andrew Jackson. He successfully engineered a genocide, trampled the Constitution and human rights, and was actively hostile to limits on Presidential power.
We'll see if 47 surpasses him. He's set up to do so. It's going to be wild to see what happens when Trump order troops to fire into crowds of American citizens.
We always seem to get this crazy hyperbole that Trump is going to be some competent fascist that's going to perform some great coup that will end the US, but in reality it always seems the real damage he does is the evil bureaucracy that erodes rights and liberties while exacerbating things in foreign policy.
Jan 6th was very flashy, but comparatively speaking, nothing really happened.
I'm not afraid of Trump's competence. I'm afraid of Trump's cult of personality and the competent people that are now handling him. We didn't elect Trump, we elected Project 2025.
Handling is a somewhat strong word here though. For better and worse, he's very impulsive and egotistically sensitive. His last administration left a huge wake of people that haven't been rehired and likely won't be.
Even if we assume his new lackeys care enough about the Heritage Foundation to attempt to implement their plan, I'd be amazed if they could corral his attention long enough to get him to sign anything in.
George Washington got to be in charge of a country that enshrined chattel slavery in its constitution for 20 years. Thomas Jefferson provided military aid to France's efforts to quash a slave revolt in Haiti. Andrew Jackson personally orchestrated a genocide against the Cherokee, Creek, and Choctaw (all of whom were aligned with american interests). Zachary Taylor put a bounty on bison, in an effort to starve the native Americans of the great plains. Abraham Lincoln allowed his western military expeditions to do basically whatever to the native Americans. Andrew Johnson started the process of letting confederate leadership be who directed reconstruction rather than being punished for it.
And here we reach Ulysses S Grant. one of America's favorite punching bag presidents because he got scammed rather frequently, but when you dig into why he was prone to getting scammed, it's because he thought it was America's duty to use its economic power to help the lowest people in society. it's hard to be mad at a guy who was trying so hard to help people that sometimes he let someone con him into thinking they deserved help.
i can keep going on how a ton of our historical presidents have sucked. i'm still personally willing to say the top 3 are Andrew Jackson, Ronald Reagan, and Donald trump
Pretty much all of them actively participated in various genocides and massacres, either directly like native genocide or Philippines or all the aerial massacres of XX and XXI century (even the one who was president for a month), or indirectly like even the "most peaceful president" Carter supported the massacres in Indonesia.