This may surprise you, but I still put GW Bush last. Only because Trump hasn’t done anything. Yes, he bungled the Covid19 response but he didn’t start wars based on lies and he didn’t authorize torture prisons and he didn’t try to install Michael Cohen on the Supreme Court. Among many other things. But Trump will no doubt change my mind.
Seems like a reasonable judgment call.
There's the undermining of the Constitutional order itself, particularly debasing and thwarting the Congressional oversight role, the undermining of science as solid guidance in policy-making, and the unprecedented attack on truth itself, with his unprecedented rate of lying, weighing heavily on Trump's scale. Not to forget his working to destroy the international institutional order the U.S. diligently helped to create, giving nations at least a semblance of stability amid their otherwise ongoing squabbles.
Maybe we could opt for a shared last place for the monstrous clown and the carnival barker?