Potentially? Trump. Factually? Bush. However, to be honest, the American political system seems to be fucked up to the point it doesn't resemble a democracy. Currently, their population suffers from this situation with poverty, addiction to drugs, a corrupt healthcare system, inability to own a home, shitty jobs, etc. So, it really doesn't matter too much which one is worse. Biden or nobody else can fix this from within. But yeah, a second term of Trump would be definitely catastrophic and would compete with Bush's levels of destruction. Right now, the only thing containing Trump is his short term period in power.