If you are referring to brute fastest, the Lamborghini NOMAD will top out at 676PP / 870hp. It is by far the fastest superGT when talking about limits.
The new DLC one, the Weider HSV in my opinion is the fastest all rounder SuperGT. It has a nice torque / power curve so it doesnt twitch, is lighter than most others (i run mine with a 50 ballast at the rear, so it handles more MR) and goes up to 40/65 Downforce. In my opinion its the best Supergt car in the 625-650pp region. (It goes up to 659pp I believe)