The FF car with the overall best stock power-to-weight ratio in the game is the Citroen Xsara Rally Car.
The FF road cars with the best stock power-to-weight ratio are, in order:
1. Spoon Civic Type-R (214hp/820kg - 261.0hp/tonne or 3.832kg/hp)
2. Trial Celica (280hp/1140kg - 245.6hp/tonne or 4.071kg/hp)
3. Ford Focus RS (217hp/1050kg - 206.7hp/tonne or 4.839kg/hp)
4. Spoon Integra Type-R (217hp/1080kg - 200.9hp/tonne or 4.977kg/hp)
This brings up two important points. Firstly, this is why I (amongst others) created the complete GT4 car database - so that anyone can download it and look up these facts for themselves. Secondly, though it's gratifying that so many people are willing to help, please do not give the answer if you do not know the answer.