If you had to have just one perk in your setup which would you choose and why? You can categorize it for different types of classes or just use one all of the time.
For example: someone who uses Assassin all of the time may not want to use that because they wouldn't get the benefits of something like Scavenger.
My choice: On a submachine gun class I would use Sleight of Hand since most of their reloads are long and you already move pretty quickly while aiming down the sight.
I would use Stalker on most assault rifle classes to make up for the slow movement while aiming down the sights, but If I had to chose just one for every class it would be Sleight of Hand.