I like the BMW M3 GTR. It's one of my favourite cars.

But even with careful applied Pro parts it falls behind other Tier 3 cars. It handles very well (to me it is the best car when it comes to high speed cornering ability), but lacks both top speed and acceleration. If you have invested all necessary skills in race to give you some advantage, you might be able to win. But if other players also have those skills and a proper tuned GT3 RS, RTR-X or CCX, you're done.

I also have the SLR, but it only has Race Tuned parts.
As for Tier 1, the Toyota MR2 is definitely the best car. I carefully choose Pro parts for it, trying to balance everything, and the result is astonishing. It might lack a bit of top speed, but that's it, really.
As for Tier 2, I'm having a hard time deciding which car I should upgrade. I have various which are quite good and all have Pro parts (Audi A1, Exige Cup, Miura), but none of them is really good. What I want is the BMW M3 E30, a friend of mine has it, Pro parts installed, very balanced and it just beats anything.

I drove it and it is an amazing car.