The RUF 3400s is by far the best handling "completely stock" (no mods, no new tires, no oil changes, etc) car in the game. It might not be the fastest but it's balance is perfection, even for an MR car and it's ability to be driven loose or with lots of grip straight out of the box is great. I find this car the most enjoyable stock car to drive and have used it in combonation with the RGT (with mods and tuning though) to clear as much of GT3 as I can (basically anything I can enter either car in)
Second would be the RUF RGT. While this is my favorite car in the game, especially in it's stock form, it isn't quite as good as the boxster (3400s). It doesn't work that well with normal tires, not as well as the boxster, but with race tires it is at the same level as the boxster as far as handling. Once you start tuning it and adding mods to it it becomes by far the best car in the game. For my driving style this car at any level of tune beyond stock can do it all. It is my swiss army knife.
The RUF CTR2 isn't so bad completely stock either. In fact at completely stock, though with race tires, it is probably at it's best stock (unless you have spent time to learn to tune it). I love this car but it is #3 on my "completely stock road car" list. I love the sound of the thing stock as well.
Honorable mention? Hmmm....the mini isn't so bad, though you are only going 50mph (hahaha) so that masks alot. The Clio road version is pretty good, though sorta bland and boring. The Peugot 206 isn't bad either as is the Alfa. Oh the Cobra is good fun stock with it's 4 speed transmission, actually one of the better stock cars, but it is much more fun with mods. The NSXs are all well behaved stock, very well behaved, though their braking is rather bad. The Viper GTS (the road one whatever that is) is pretty good though again it needs parts to become very good. The Subaru 22b is pretty good, for someone looking for a stock normal road car for the PD cup as a noob this is your best choice.
The Tommy ZZII is a good car, I use it for my "race car" (since that is what it really is in GT3 and since I find most of the LM and prototype cars in GT3 rather boring to drive)...but it isn't a stock car. It has a parts in it which disqualifies it for normal car races even when you get it new.
and fun to drive.
