Clue: it’s not the PSP one.
Why does Kazunori Yamauchi, creator of Gran Turismo, get special dispensation from Sony when he makes a game? Because each main Gran Turismo release has sold roughly 10 million copies – or more.
The best of the bunch is Gran Turismo 3 A-Spec, released 2001, with a gargantuan 14.80 million copies sold worldwide.
Gran Turismo 4 and the original Gran Turismo are almost neck-and-neck behind GT3, with 10.86 million and 10.81 million sales each, respectively.