Neither the Bullet GT or the F620. The F620 is just a SuperGT clone, and the Bullet GT is just an overpowered flimsy P.O.S. in my opinion.
For me my favorite supercar is the Turismo. Excellent top speed, best brakes in the game, and great acceleration. The fact that it's not in traffic in TBoGT multiplayer means that the Turismo is now a rare vehicle in free mode. You can park it in a parking spot, and it'll stay there. If only it had a rear wing option...
And the title says "which supercar in TBoGT is your favorite", not "which supercar EXCLUSIVE to TBoGT is your favorite", just to clear things up.