I used the R34 GTR fully tuned to the max with soft racing tires etc etc to easily win almost all races that it was eligable for..then I moved onto a fully tuned Stealth Mazda 787B.
If you want to max your grinding but still not make it feel like grinding I suggest this (for bspec btw)
Do the dream car championship in an fgt/x1 (or any car that will dominate) and let bob do the whole championship. Leave it and just check back every 20 min or so to start next race. If you win the first four just start at the burg then exit after the race starts to save 25 min. At the end you get the bonus 500,000 credits for Golding the championship. So basically after about 80-90 min you have earned nearly 1 million credits. The races are all roughly as long as grinding Indy anyway (for bspec) and the bonus cash makes up for it anyway.
Happy Grinding
Edit: You can do the same for A-spec varying the cars you drive to keep it interesting
I try my best not to grind, but last night I was at 9 million credits so I grinded around Indy so I could get the multi millionaire trophy. I used my ZR1 RM.