This. And it's incredibly easy if you reload it until you don't have a McLaren, Zonda, or Ford GT starting up front.
Edit to add:
The Mt. Aso race in the recent FR seasonal and the Cape Ring race in the FF seasonal are also very good money-makers, but you do need to use a slower car to get enough bonus, and I still think they take a bit longer than that Nordschleife race. And of course there's always Time and Drift Trials for a quick million if you haven't done the latest already. So a brand new GT500 car is never more than a few minutes away.