11 For zombies 9 for MP, i find 11 a good enough speed on zombies to turn around and check behind you fast while still being able to aim well. 9 I use on MP so its a little easier to aim far away.
Currently mine is set on 9... it works for me.
I also still use Aim Assist. I have friends who encourage me to turn it off..... but so far I haven't been able to ditch those training wheels