Well, run whatever you want but the only relevant mon I can think of(barring scarfers) that's faster than Greninja is Mega Sceptile, which requires more than 176 to outspeed and isn't very common.I forget the actual relevant mon but it is what most people run.
That being said, I gave incorrect EVs in my previous post. Mega Swampert only needs 52 in Speed to outspeed 252+ Greninja in the rain.