The question is unanswerable.
Pokémon lore, especially move and Pokédex description, has always been very exaggerated since it's supposed to appeal to children. There has never been any proof that the grandest claims found in the Pokédex are true. For example:
Magcargo's body temperature is approximately 18,000 degrees Fahrenheit.
You'd burst into flames if you so much as looked at one.
Alakazam's brain continually grows, infinitely multiplying brain cells.
Alakazam's head has yet to explode.
[Machamp's] four ruggedly developed arms can launch a flurry of 1,000 punches in just two seconds.
125 punches per arm per second.
Manectric is constantly discharging electricity from its mane. The sparks sometimes ignite forest fires. When it enters a battle, this Pokémon creates thunderclouds.
Manectric does not, in fact, create thunderclouds or discharge constantly.
[Cubone] always wears the skull of its dead mother, so no one has any idea what its hidden face looks like.
I've bred Cubone before, and my Marowak still has a skull, and is definitely alive.
When [Victini] shares the infinite energy it creates, that being's entire body will be overflowing with power.
We've got a being that can produce infinite energy. Not a huge amount, not enough to obliterate the universe, it's literally infinite. I don't need to tell you how ridiculous that is. If any Pokémon is the fastest, it's Victini.
And finally, Quick Attack's description:
The user lunges at the target at a speed that makes it almost invisible.
I guess anything that learns Quick Attack is the fastest Pokémon in existence, if you want to trust the lore, but I prefer this move.