I wouldn't vote for Jimmy Carter, but he strikes me as the most morally upstanding and honest politician in living memory.
Interestingly, I've read in several variously sourced places that he is actually a pretty mean, petty person...a fair number of early supporters and staff on his presidential campaign bailed out because of what they saw as his totally hypocritical behavior. He is apparently not particularly well liked by the current "President's Club" either. Bush Sn, W, and Bill Clinton all get along famously, and W/Obama, Clinton/Obama relationships are also solid if occasionally prickly (no knowledge of Obama/Snr dynamic). But none of them apparently like Carter much at all. On the other hand, maybe it's Carter who is the nice one and all the other presidents are assholes? Hard to know from the outside.
However, personally, I would have totally said Jimmy Carter if I didn't know that info. My sense is perhaps he's fairly honest in his public dealings, but just not very personally likeable.
Also people can be upstanding and honest in some areas but not others. I know lots of people who strive to be honest in their professional lives...indeed, who pride themselves on that, but then blithely cheat on their spouses and feel justified in doing so
Perhaps we should separate public and private behavior? It's hard to say. Al Franken is another who seemed honest, but with recent events, I'm really wondering. Paul Ryan strikes me as honest and probably fairly tolerable personally (though he's reported to be total brown noser). However, he has some seriously sociopathic policy beliefs IMO, which I view as actively evil. So would I think of him as a good person, or not?