Ha, good question, because it has just changed. Having the fastest computer is now like a competition between major nations. And China has just got ahead (a couple of months ago) with a computer called Tianhe-2.
If you like numbers, I found this:
The Tianhe-2 is capable of sustained computing of 33.86 petaflops per second, which is the same as 33,860 trillion calculations per second, according to the semi-annual TOP500 listing of the world’s fastest supercomputers by a group of 500 experts.