Another vote for the RM ZR1.
The ZR1 has earned me more credits and prize cars than any other car short of the FGT. It will win you Like the Wind to get the Minolta car (this will be your other big winner for everything you can't do with the ZR1) and it will easily win the American series championship in Extreme for easy credits grinding among many others. It's also good for the current seasonal race that has a 1.5 million credit prize which will essentially make the car free and leave you with another million credits to buy another car, the smart thing to do would be to save up for the FGT at this point since the ZR1/Minolta combo will leave you plenty of options for A Spec and B Spec. Before buying anything, be sure to check the prize cars list to make sure you aren't spending a lot of money on something you can easily get for free (like the Zonda R).
Almost all the other cars you can buy at a similar price point will only be good for one or two races that aren't worth doing more than once to get the gold and prize car.
