My favourite isn't really a 'conspiracy theory' in the truest sense, it's a or from a movie.
The story goes that Elvis, way back in the late 60s, wanted to have a few years out of the spotlight and switched places with a popular impersonator.
The real Elvis moved to a trailer park, made friends, enjoyed being a nobody.
The fake Elvis went off the rails, started getting fat, being over the top with spending money and generally going a bit mad.
Unfortunately, the fake Elvis died from too much indulgence. The real Elvis lost his evidence in a BBQ accident.
So now Elvis is living in an old folks home remembering his days being the king of Pop, everyone around him just thinks he's nuts.
In the same movie, JFK is still alive but his mind has been transplanted into a black man and there's an ancient Egyptian mummy stealing the would of the old folk... Which he takes by sucking them out the arse hole... It's a great movie, check it out..
It's called:
Bubba Ho-Tep