Finding a rival for this is almost impossible.
You could make the attempt at comparing the Koenigsegg One:1 against, say, the Bugatti Veyron SS. Heck, why not even toss in the Hennessey Venom GT? But when you look at the gritty details, it becomes abundantly clear that the One:1 has no rivals, at least not yet. Bugatti and Hennessey hitting nearly the same top speed as the Koenigsegg doesn't qualify them for complete equality. The Veyron is a heavy sucker, tipping the scales at over 4,000 lbs. Call it a beached whale.
The Hennessy weighs in at 2,743 lbs. And the Koenigsegg? Just under 3,000 lbs. In kilos, that's 1,341 kg, thus the car's name. It has a 1:1 power-to-weight ratio (1,341 hp). That right there is an incredible achievement. And until someone else manages to accomplish – and surpass – what Koenigsegg has just achieved, the One:1 is truly in a category all of its own.