The Ariel Atom has always been the ultimate power-to-weight geek's car. A car with a humble four-cylinder engine that snaps off a sub-3 second 0-60 time shows that those geeks are onto something. The Atom 3 is the latest incarnation of the Atom and it is also the most powerful. It's also the biggest, but you would need superhuman powers of perception to notice (although if you're determined to try, check the back, the Atom 3 is the only one to have dual exhaust).
The tuned Honda K20 engine comes with three different power rating options: 200hp, 245hp or a supercharged version with 300hp. With a curb weight of just 1350lbs, that means the supercharged car has just 4.6 pounds per horsepower. Since the Atom pretty much has no body at all, it might lack the sexiness of an out-and-out supercar, but there is simply no way to get more speed for $50,000.