The A-spec Challenge 4 on 650pp Nurburgring gives about 1million credits in 6mins. Easy. Many different cars can be used to win this.
The Ferrari F1 seasonal on Nurburgring gives about 4million credits in 14mins if you can win with 770pp. Or 3million for 800ish I believe. I usually run it at 780 which allows for a few minor mistakes to be made and still can result in victory.
Also, we have an edit button. Use it instead of posting multiple times in a row. Also, there are many threads on the best ways to get money quickly. Use the search function next time.