Ignoring the previous two terrible answers below, it would depend on what kind of computer you are talking about. In general, Supercomputers are the fastest above everything else. Super computers are a conglomeration of multiple individual computers which are networked together and controlled by a master computer. By networking all the computers together, you can distribute one task on the main computer to all of the other computers evenly. This divides the stress of certain tasks like 3D rendering, and long mathematical equations, and code breaking in order to decrease the amount of time it would take to finish the task on a single computer. Currently, the fastest super computer in the world is the "Cray Titan" which resides in Oak Ridge, USA.
As for home computers, Alienware is renown for building over-the-top gaming computers which, for a price, will give you the maximum possible for commercially available technology. But for the most part, almost all major computer companies make a laptop/desktop that will come with what is the current max specifications for top performance.