The GT-R is a real powerhouse, one that also understeers quite a bit, but it can be brought back in line with a bit of tweaking.
The SLS is sublime, it has the straight-line performance and cornering ability that I want out of a race car.
The BMW is like the Mercedes, but it has a shorter wheelbase, so it can get frisky if you aren't careless.
The R8 LMS ultra (Spending over 1.5 million credits on a GT4 car is silly.) is very easy to get to rotate, but can get out of shape like the BMW.
I can actually effectively use all of them, but my choice is the Mercedes-Benz SLS AMG GT3. Now if they would only sort out the sounds of it and the BMW...