In his essay on President Obama’s first term, James Fallows dismisses Obama’s conceit that he would prefer to be “a really good one-term” president than a “mediocre” president who served two terms. “The reality,” Fallows writes, “is that our judgment about ‘really good’ and ‘mediocre’ presidents is colored by how long they serve. A failure to win reelection places a ‘one-term loser’ asterisk on even genuine accomplishments. Ask George H. W. Bush, victor in the Gulf War; ask Jimmy Carter, architect of the Camp David agreement.”There is of course a bit of sleight-of-hand here: being a good one-term President might differ from being perceived as a good one-term president.
Example: I'm not exactly as screaming groupie for Jimmy Carter, but I do think he was a good deal better president than (most other people's) perceptions. I suppose your answer would also depend on what you want out of a president. I've always had a certain affection for my fellow New Englander, the cuddly Calvin Coolidge. I don't actually think he was a particularly good president but I'd have to grant he did exactly what he told us would, which is also what he wanted to do. And as the fellah says, if you like this sort of thing, this is the sort of thing you will like.
But here is the great counter-example: James Knox Polk--still our "our least-known consequential president," as adjudged by those worthies at The Daily Beast (though perhaps a bit less well known than he was a few years ago, thanks to a brisk and instructive recent biography).
In the celebrity sweepstakes, I think Polk has not one but three strikes against him: first, that he served only one term. Second, that his achievement (and purpose) was to establish an empire, and we are still a little skittish about the empire thing. And third, he died, just after leaving office. We can only wonder how things might have gone differently had he hung around long enough to write a self-justifying memoir. Granted, neither Kennedy nor Lincoln needed the opportunity to explain themselves. But we can think of so many examples--Hoover and Nixon come quickly to mind--of presidents who left office in the shade and thereafter clawed their way back to--well, perhaps we can say "clawed their way back to mediocrity." Surely Polk, had he chosen, could have done at least as well.