I usually play crucible and use snipers in my special slot.
I use kontrol freeks, a long one on my aiming stick.
Also I play on a 24" monitor
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The best sensitivity to play on would probably be the highest one you can acclimate yourself to. If you can play on 10 without your aiming being erratic, than play on 10. If you can only control your aim with your sensitivity on 8, than play on 8.