I haven't yet tried all of GT3 cars, but SLS and all three BMWs really surprised me enough that I spend quite some time driving them. M6 felt like a cheat car, it's very stable and at the same time very good at cornering, something I didn't think can come together in one car.
Before the physics update was the Audi R8 and the old Porsche 911 my favorite. Now I feel very well with the AMG and BMW M6, they are so stable and I can drive with that consistant laps what I missed with Porsche or Audi. I love the sound from AMG and BMW too.