2 for ARs, 4 for SMGs and shotguns and 8 for snipers. I've changed it for different gun types since BO. Seems to work for me.
**haven't read other responses.
Depends on tv size and if you are using wired or wireless controllers. I play on a 55" wired, so a 7.
20. It's great for wanting to turn around quick when you need to react fast. For those playing at ridiculously low numbers please do yourself a great favor and invest in a pair of FPS Freeks. They are worth it and males 20 feel like 7. No joke.
I play on 4 because I am a slower layer, playing on high sensitivity can have benefits and negatives for example the higher sensitivity could make you miss where as the slower sensitivity is more controlled but you may not be able to react to some players in time both have their positives and negatives.
Currently on 15 to 17 I try and change it up on a regular basis it helps to be able to snap turn, I build it up slowly until I am proficient on a certain speed then increase it by 1 at a time.
6. I simple reason. Miss one or two shots in this cod and you're done. I see how higher helps, but I am not able to hit long range shots on high, nor do I get how these guys can see what's going on on their screen with it jacked up to 15- 20. I wouldn't be able to focus. lol.