Is this the world's most valuable car?

No. It just happens to be the most valuable car to be put up for sale recently. There are more than a few 250 GTOs out there, and there are much rarer, more valuable cars which may never be sold at auction.

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If the Bugatti Royale Kellner Coupe (above) which sold for $8.5 million in 1987 ever went to auction again, it would immediately jump to the top of the list ... for a while. It was the most valuable car in the world for 21 years before being swamped by the marketplace, with five new records in six years. There will be more records and they won't all be 250 GTOs.

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In May 2010, Gooding & Company handled the private sale of a 1936 Bugatti 57SC Atlantic for more than $30 million, which at the time, was believed to be the highest private price ever paid for a car. Neither the exact price nor the buyer were disclosed, and it's interesting to note that so beloved are many of the cars in the collector stratosphere that owners would often rather see the car go to a good home than to the buyer with the most money.

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The Atlantic was derived from Bugatti's fabled prototype Aerolithe Electron Coupé, which was shown at the 1935 Paris Auto Salon. Only three Atlantics were made, and only two still exist, with one owned by fashion designer Ralph Lauren. That's an Atlantic in period above – only slightly less rare than a unicorn.
Several Bugatti Aerolithe Electron Coupé concept cars are believed to have been built, but no-one knows where they are, or at least they're not telling anyone. If one of those surfaces one day, it would command a world record price.
So while the GTO is the world's most valuable car ever sold at auction, and also believed to be the most valuable car ever sold privately, there will be other makes and models that take those records in time.