William Henry Harrison.
The dudes biggest accomplishment in office was dying.
But that, being the first case, created the vice president precedent.
But that wasn’t an accomplishment of his own accord.
The thread is "who was the most inconsequential," not "who accomplished the least."
Well, I can forget mentioning Calvin Coolidge then. He was the person who mostly presided over the events that led to the Great Depression.
This has long been quite the oversimplification that GOP Presidents of the 1920s practically single-handedly engineered a worldwide Great Depression through their conservative economic policies alone, lol.