Every Gr3 car can win it with the normal difficulty on stock specs and Racing Hard tyres, even the abysmally slow Z4 GT3. Depending on the car either 5+5+5 laps for both fuel and tyres, or 6+6+3 for fuel and 6+9 for tyres.
The fastest for me has been the Viper, after that in no particular order (using alphabetical here) the Aston DBR9, Ford GT LM Test Car and the McLaren F1 GTR. In other words anything that is clearly above the "normal" GT3 specs of ~550-600 bhp and ~1250-1300 kg. Honourable mentions go to both AMG models.