I think Carter gets a bad rap. He had the misfortune of being a global-minded, socially concious president at a time when all people wanted was their gas prices lowered. People tend to blame him for the entire Iranian hostage crisis, but then overlook the Camp David accords. And with all the recent indications that perhaps there were some behind-the-back dealings to make sure the hostages weren't released on Carter's watch, I really can't fault him for that either. But looking at everything Carter has done since leaving office -- well, if he had served Reagan's term, we'd be calling him one of our most beloved presidents ever.
It's easy to pick on Nixon, but then you'd be overlooking his relations with China and Brezhnev. Watergate looks positively innocent these days. Ford's an easy target too, but who really expected anything from him? And Clinton will probably settle somewhere in the middle -- even if it did turn up something, the rabid witchhunt waged against him for eight years is bound to make the Republicans look worse in hindsight.
Reagan is pretty unassailable in most people's minds, but I think he was basically little more than a cheerleader for eight years -- after the chaos of the 60s and 70s, he was the grandfather telling us that gee our country was swell and don't you tell us any different. Our generation's version of Eisenhauer, basically.
My least favorite would have to be Bush Sr. He couldn't wait to wade in and proclaim Reagan as the winner of the Cold War, even though Kennedy and Nixon did a hell of a lot more to bring about it's eventual end -- Reagan basically was the orderly brought in to keep watch on a terminal patient, but to hear Bush tell it, Ronnie was over there knocking down planes and turning over tanks. His fingerprints were all over Iran/Contra (a MUCH more grievous crime than getting a blow job in the White House, if you ask me) and he skated away untouched. And he proclaims Saddam Hussein to be a new Hitler yet conveniently leaves him in power once those precious Kuwaiti oil fields were safe. So I guess Hitler is tolerable as long as we get cheap gas....
That said, George W. has ample potential to replace his dad, given time.