Non-legendary - Definitely Ninjask, like Cowrie said, base speed is the highest of the normal pokes there. Plus it just looks light and fast. Plus its got Ninja in its name! XD W00t Ninjas.
Legendary - While Deoxys is the fastest according to the game, I dont think it would be as fast, say on the cartoons, as the legendary dogs would.
And did anyone consider that once Porygon/P2 enter cyberspace (as only they can do) theyd be travelling at the speed of light?