Ninjask is the fastest Pokemon that is allowed in OU. With a positive nature and maxed speed Evs Ninjask has 460 speed.
The fastest OU Pokemon is Jolteon. Witha positive nature and maxed speed Evs Jolteon has 394 speed.
There are a lot of scarfers who outspeed those two Pokemon. Ninjask has Speed Boost which is very useful.