What is the best rated (in terms of ballistic protection) soft armor available?

IIIA

Custom is right, IIIA is the best. If you're useing plates, read the back of your plates carefully. Alot of people see that it says Level IV in big letters, but don't read the fine print. Usually it's level IV when in conjunction with level IIIA soft armor (unless you actually have level IV stand alone plates). If you didn't have the soft armor behind the plates, the round would get stopped by the plate, but you would get killed or seriously injured by the backplate deformation. So, check you gear carefully. Your life may depend on it.