In my opinion the Cerbera LeMans edition is the best race car (looks and performance)
However the best non-spec (production) car I had in GT1 was the Aston Martin DB7:
In addition to looking good it has an awesome cornering capacity: just tap the brakes and turn, and it swings to one side and immediately stabilizes itself!!!
It has a decent acceleration too, though not as fast as the Viper.
I put the following on my DB7:
Turbo Stage 1
Racing Muffler/Exhaust system
Computer
Engine Balancing
Port
Full Racing Transmission
Triple plate clutch
Racing (lightweight) flywheel
Carbon fiber driveshaft
Weight Reduction up to stage 3
Suspension:full racing
Racing soft tires.
and ended up with a 414 BHP,3200 LB car
Took it to the GT cup (for License A) and won against cars with much higher power to weight ratios (And won my first Lemans Car in the game!!!!)
I also used it to win the Anglo-American Challenge, but then I stopped playing the game for a while. (Yes, it beat the Vipers, Corvettes and Griffiths overloaded with mods)
Hope to continue soon...
But the DB7 is awesome, it is worth 150 grand to spend on!!!