So far I've only really raced the F-Type GR.3 which I've found pretty good. If you don't use TCS it can be a handful though. In GT5/6 I always had TCS set to 0, but I've found that in GTS it's very difficult to use the high powered RWD cars without it. That being said, I now usually keep my TCS set to 2 and this makes the Jag very competitive against the other GR3 cars.
I've also seen people having great success with the GR3 Beetle, oddly enough. Overall, I think all the cars are fairly similar in performance, kind of the point of the grouping.
If you haven't already, I recommend breezing through some of the Campaign Mode. You're practically guaranteed at least one Gr.3 by the end of it.
I haven't experimented much in the full game with Gr.3, but I was partial to the AMG GT3 in the beta. It's a pretty good all-rounder, and being FR with a nat-asp engine, it's slightly more forgiving than some of the others.
The 911 RSR seems to be strong at Maggiore right now, if my time in races is any indication. I've managed a win there in it, but it requires a different approach than the AMG.