Dream car, Tokyo(numbers) 18 minutes in a formula GT....16k xp for B-spec. and around 90k money.
If just pure money. then A-spec, american-indy.
EDIT: And I think best MONEY for B-spec would be the german touring car at nurburgring. 15 minutes, about 100k.........but the Tokyo(numbers) in the dream car is only a few minutes longer and WAY more xp and about the same money.
if you're at level 26 then you should be more concerned with xp than with credits so just keep doing the endurance races. that way you get big xp payouts without having to restart a race every 20 minutes.
If you want money and don't want to actually play B Spec I found the following works well for me.
High speed Ring = About 10 minutes for $50,400
Not sure on the EXP but this is a total autopilot race you wont need to monitor.