I HATE these kind of threads. There is no "best" look sensitivity. Its all up to personal preference. Come on use common sense.
I realize that this is a 6 years old post but if I were you I would start at a 5 or 6, then as you get more experienced at the game, increase it as you get better. Once you get to the point where the sensitivity is too high, go to the next lower one e.g. is 10 is too high, but 9 is good, use 9. Do this for all Call of Duty games that you play and you will find your sensitivity in about 2 months, but it's worth the 2-month process.