Sort of depends on your level. My two favorites so far were the lv. 13 (I think) brittish lightweight series using the Top gear track. The race takes approx 9-10 min and you get 24k and 1700 exp if I remember correctly. Its also an easy win for my race modded elise.
Now at lv. 19, i've started doing the b-spec polyphony cup. It's a race series, but only 3 races with about 25 k for each win, and 85k for the series championship. I forget the exp amount, but its alot.
B spec isnt good for grinding credits since all the races are longer than the A spec version. I'm glad they did this for balancing. You should earn more for your own driving. The special races are usually the best money but they're tough since they assign you a car. The one exception the the rally race where you're only restricted by HP. So just get the lightest car you can with the maximum hp. The Rally races are also quite short so it's better money than the Daytona ring.
aling0 posted... B spec isnt good for grinding credits since all the races are longer than the A spec version.
On the contrary, dear member--B-Spec is perfect for grinding. You only have to come back every 8 or so minutes to restart the race. So you can do your own thing while your slave mines credits on your behalf. It's perfect. Fair.