My apologies for resurrecting an old thread, but I'd assume that's preferable to opening a new one.
I've been on a hunt for the past few days to find the best sounding engine in the game, and although my search is far from over, I've got to say that the Nissan Fairlady Z has one of the most impressive sounds I've ever heard a car (real or otherwise) make. It's also one of the only cars I've found that's sounded great stock, with a semi-racing exhaust, and with a racing exhaust. It's powerful, it's loud, and it's pretty fast as well. I can't remember the exact name of the car (away from the PS2 at the moment), but I believe it's one of the 'Full Models' available in Nismo, and it's silver with a red/black decal on the hood, and it comes with black rims. I'd strongly suggest you check it out if you've got a bit of extra coin.