I would suggest the Holden VXR8, but frankly its looks give it away.
My idea of what is asked here is what car would give a much faster car a good run for its money in stock form. It should be a car that is completely conventional looking and when compared with a sports car. It should look like a car that is slow and normal, yet when given some stick, can smoke a tuned car.
My best guess would be the Alfa Romeo 159. But then again, it looks like a performance motor.
To be honest, I think the best would always be a Ford Crown Victoria. I looks completely conventional, large and heavy that you'd hardly expect much performance from it. But then again those who watch alot of action shows know that the cops won't use anything that is slow for a high speed chase...
Looks like this is completely difficult question to answer.