Frankly, not sure where to put this so I imagine it will be put in its place shortly by the mods.
What person in the last century in your opinion has cratered the most spectacularly/substantially?
Mao?
Paul McCartney?
I can’t answer the century bit, but I can try the past several decades…
Last decade, John Edwards. Guy had a chance to make it to the VP slot with either Obama or Clinton. His scandal was simultaneously the most boring and most stupid affair in political history. No matter how much of a bitch your terminally ill wife is, you can’t be screwing a space cadet behind her back and then having a kid afterward!
Might have gone down as this generation’s RFK. I don’t think the guy has a shot of having any prominent position in public life for a long, long time.
And on the international scene, how about Winnie Mandela? Prior to and right before Nelson’s release, she was one of the most admired women in the world. Maybe a decade later, she’s on trial for corruption, accused on sanctioning murders, and convicted of fraud. Remember how Sondra and Elvin’s twins on The Cosby Show were named Nelson and Winnie? I would have loved to see how that shaped out. 
Further thinking about it, how about Stalin? At one time, dude was either the most, or second most powerful man in the world… he was considered a monster even by Politburo standards. Imagine if FDR or Churchill were “erased” from our cultural memory… that’s what happened to Stalin ten years after his death.
This is fundamentally different, though: Stalin was always a monster, it’s just that after he died, the people he tyrannized in the government saw no reason to continue the cult of personality and de-Stalinized the Soviet system. His decline wasn’t caused by any misstep of his; all that happened was that he died and was therefore unable to send detractors to gulags.
O.J. Simpson?
As a group Firemen in the U.K.
It used to be that we all admired these men and women (myself included) for going out and risking their lives everyday.
But when they took industrial action their work was investigated and it was found that actual firefighting was a relatively rare occurance and that they spent a lot of their time on sleeping watches or playing snooker while awaiting to be called out.
It was not uncommon for them to have second jobs because their first jobs didn’t wear them out.
Also they were payed quite generous wages compared to the norm.
Almost overnight they lost their heroic status amongst much of the general population and some of their most scathing critics were members of the armed forces who actually did and do risk their lives daily for much lower pay.
Personally I have no axe to grind with firemen, as they actually do on occassion risk their lives (but not very often), but their reputation in the past was so "bigged up"it was inevitable that it would cut down sooner or later.
Nikita Khrushchev went in a few days from being the second most important, powerful person in the world to being a powerless, irrelevant, ignored “retiree” wandering around his dacha. An american president who leaves office does not experience such a change in his status or statue. (It was almost true in Nixon’s case, but he had already lost the respect of most people before he resigned even though he still had enormous power.)
Moved GQ --> IMHO.
Clear it depends upon how you define the terms, and what weight you place on the three metrics.
Relevance, Importance, Respect.
If you consider POTUS as probably the single most powerful individual on the planet, which isn’t unreasonable, then every outgoing US president pretty much tops the relevance and importance metrics as crashing out. One day you are commander in chief of the worlds largest military power, and the next day, a B grade talk show guest. In terms of respect however, that is much more variable. On this basis you would probably rate Nixon as having one of the greatest crashes. Resigning as US president and becoming a political pariah is hard to beat on the metrics.
Ted Kennedy?
No more so than any famous person declines in importance after their death.
In the entertainment world, I’d suggest Arthur Godfrey. He went from being one of the most famous and powerful entertainers in the country to a virtual nobody within a generation.
15-20 years ago, Mel Gibson was a well liked, much respected Hollywood mogul; I don’t know what his reputation with his industry peers is like right now, (I am guessing not great, as he has been heard on tape bashing gays, Jews and African Americans " i.e. “raped by a pack of niggers”) but he is generally seen by the public at large as a physically abusive, emotionally unstable bigot.
(I also think that Hippy Hollow’s choices of John Edwards and Winnie Mandela are spot on for this subject. John Edwards could have easily been a major force in national politics had he conducted himself differently)
Howard Dean’s campaign in 2004 seemed to be going well, with lots of grassroots support, a big campaign warchest, and early front-runner status. But then he came in 3rd in the Iowa caucuses.
And then this happened.
The OP is looking for people who have “cratered.” I would say being one of the three major world powers at the end of WWII, arguably ending Hitler’s war machine, to being erased from history is pretty spectacular. For a fairly closed society that doesn’t like to admit to mistakes, that was huge. Dissidents in the Soviet Union were dealt with harshly up until the end of the empire, so for a group of such tough actors to say, “Hey, that guy was really off the reservation,” that’s pretty huge.
Also, while we’re at it, Nicolae Ceauşescu has got to be in the running.
In the world of business, it’s got to be hard to beat Gerald Ratner.
Hmm… would Thomas Crapper be regarded as a success or a failure for this thread? 
Michael Jackson’s on up there. I have an old copy of Jane and Michael Stern’s “Encyclopedia of Pop Culture” from 1992 that starts the entry on Michael Jackson with something like “Michael Jackson is probably the most universally beloved person on Earth.” He suffered from a little PR trouble subsequent to that writing.
I think his own people always hated him, but were too afraid to oppose him.
Go back ten years and take Gary Hart. Telegenic, articulate, new-style liberal, odds on favorite for the 1988 Democratic nomination for President. Rumors start to circulate about allegations of marital infidelity. Hart challenges reporters to put a tail on him, 24/7. That night, reporters see a woman sneaking out of his house. His wife defends him. Two days later, a photo surfaces of him and a T-shirt clad Donna Rice sitting on his lap. Three days after that he ended his candidacy.
Are you saying these are paid firefighters who have earned scorn because they’re not busy fighting fires every single day? Because unless there’s a big wave of arsons decimating the city, there’s really not much else they can do but hang around the fire station, on call… Fire departments here are having a hard time getting volunteer firemen to sign up because everyone is working two regular jobs already just to get by. So they have to pay firefighters, and the county budget is getting slashed, but what are you gonna do? Someone’s got to be available and ready to go when a call comes in, whether it’s once a week or twice a day.
On the other hand, idle firefighters have ample time to write cookbooks focussing on chili, and posing for sexy calendars!
Britney Spears who was at the top of her game almost exactly ten years ago.
But I wish to draw your attention to Oliver North. Ollie was the darling of the GOP, so he ran for Senate. The Democrats splintered with a black guy and a the former governor. The governor swore he only got a back rub from Miss Virginia. If he did, he was an idiot. In any case, Ollie still lost. Further now that the Tea Party is big, do you see Ollie?
Without a doubt one of the greatest declines in American politics.