It depends on how you define it. By powerful do you mean highest engine output? Or the fastest quarter mile? Fastest 0-100 time? Highest top speed?
Are you speaking in terms of production cars with the most power off the showroom floor? Or are we taking into account modified cars as well?
If you're talking purely in terms of engine output, it would have to be the HSV W427. Around a track I would have to say an Elfin MS8 or T5 would be pretty quick.
But of course these get blown away when you start talking modified cars.
