From someone who has all shotguns gold and uses them often, I can say that the Remmington can be pretty OP in most peoples hands. The KSG takes some good aiming so that isn't OP. The S12 is harder to use effectively, and requires long barrel and laser sight to be effective. Even then it still very rarely one hits, and to get one hits requires you to hug people or sneak up from behind and get close. The m1216 is pretty powerful but not as powerful as the Remmington. It takes a bit of getting used to but once you get familiar you can kill most people in close range within 2 shots. This also requires the laser sight and long barrel however.
Remember that this is just my experience with the weapons though. Also if you're looking to gold the Shotguns, go Hardcore. It will save you plenty of time, and I surprisingly pulled nice positives even though I barely play Hardcore.