Ok for the choice scarf, Adamant nature, 252 EVs in Speed, and 2 IVs in Speed he would have 405.7 Speed. So he has just enough speed to outspeed Jolteon but is still outspeed by Accelgor/Electrode. That is pretty good. Though neither of those will be found in OU(at least not often), Scarf-Hydregion will still outspeed. The secondd one is outsped by most sweepers so I recommend the choice scarf. Personally I usually go Jolly with Dragon-dance, that way he can speed up even more and boost his Attack to even higher levels, but if you do not want to run D-dance go with the Choice-scarf.