Underrated:
Chaparral 2D Race Car '67
Easily the fastest car for it's HP level. Detuned to 210-220HP, it's still capable of 180mph. 500HP, roughly 230mph when tuned right. Then if we go back to GT4, at just under 1,000HP, it's still capable of 300mph... Without nitro.
As for acceleration, if you're racing on smaller tracks where your gear ratio is much smaller, this car is a rocket off the line and exiting corners. Completely different story on bigger, faster tracks, where your large gears will slow you down a little.
Performance wise, at 771kg, it's super light weight and corners with the best (LMP/LeMans). Stops on a dime too.
The only real down side of this car is that it has 3 gears, making gears ratios big. Say this car had 5 or 6 gears, it could easily be one of the best cars in GT5.