I put Obama, L. Johnson, F, Roosevelt, and Wilson on there for the active damage they did to our country. I put Buchanan on there simply for doing nothing to stave off the worst crisis in American history.
He was the only American president who was a "lifelong bachelor." See a pattern?
Once you say that saving the Union was important, you have to admit that merely delaying the Civil War was important because the economy of the North was growing so much faster than that of the South.
If the South had seceded in 1850 the North wouldnt have had the logistic capability to defeat the secession, and might have realized it (or quickly been taught it) and come to a modus vivendi with it.
In a certain sense just doing nothing in the 1850s was success.