Him or maybe Pierce. William Henry Harrison didn't accomplish much in his one month--his only presidential decision was putting Tyler in the office, who basically destroyed the Whig Party.
Anyone who thinks Bush or Obama is the absolute worst needs to read some 19th century history. They aren't the best, they might be unimpressive, but the floor is pretty low for the worst.
The measure of a good or bad president should be their legacy of what their policies left us. In that respect, Bush and Reagan are quite far down the list. And few presidents really deserve to be in the positives. We've had a lot of mediocrities.