Normally I'd say an adult dog, but considering the number of pit bulls from dog-aggressive backgrounds (and I mean breeding here, not abuse), I think that in this case your best chance of getting a dog who's not dog-aggressive is to buy a purebred American Staffordshire from an AKC breeder who a) acknowledges that dog-aggression is both an aspect of the breed and a problem and b) makes every effort to get puppies who aren't dog-aggressive.
As people have said here before, AmStaffs at shows interact peacefully with many other dogs as a matter of course; they are probably the least likely to be dog-aggressive.
Too many dog-aggressive dogs are selective; they tolerate small dogs and hate big dogs, play well with black dogs and try to kill white dogs. True of any dog, yes. But when pit bulls get serious about hurting or killing another dog, it's very quick and very likely to end in death or serious injury. That would make me pause before adopting a pit bull from an unknown background into my home if I already owned dogs, especially if the resident dogs were not pit bulls. There would just be too many variables for my peace of mind - the breed's tendencies and abilities, the unknown background, the size/power discrepancy, etc.