There's no single definitive answer to "the most expensive car in the world" because prices can vary wildly based on customizations, condition, and who's selling it at any given time. However, cars frequently cited as contenders for the title include various ultra-rare and bespoke vehicles, often one-off creations. These often reach prices well into the tens, if not hundreds, of millions of dollars at auction.
Therefore, instead of naming one specific car, it's more accurate to say that the title of "most expensive car in the world" is frequently contested and changes depending on recent sales.
