I would say :obama-socialism:
Some might argue its Bush Jr. but Obama destroyed so many nations that his death toll arguably surpasses Dubya's once you include the Ukraine war. One top of that, the Iraq war sparked the largest protests since Vietnam, while Obama's destruction of Syria, Libya, Ukraine, Venezuela, etc. went largely unnoticed by the public. Obama also single handedly annihilated all progress made by Black and Hispanic communities when he bailed out billionaires and bankers. What's worse is that the psychopath is largely popular, worldwide, and libs think that his biggest scandal was wearing a tan suit.
Depends how modern you mean. Bush Jr might be the worst because of the war on terror that he started. The other answer is probably Reagan who changed everything economically for big business and workers, but that might be further back than you're thinking. To counter the Obama point, everything he did was a continuation of what Bush Jr started, as far as foreign policy goes. he got us involved in the middle east again which brought so much death.