The 1st shotgun you get is the best for begginers and the last one is the worst. Once you get good at the game the KSG is the best shotgun. Since the KSG fires a slug and requires expert accuracy at range I suggest waiting on that one.
R870MCS - Easiest shotgun. works well ADS as well as f-rom the hip. Limited Range
S12 - One of my favorites. It reminds me of the USAS from MW3 except it's not auto (I didn't use the USAS auto EVER). Keep it close.
KSG - The quick scoper of shottys. Once you get the hang of it, it'll give you streaks. I don't think it's consistent enough to be among the best.
M1216 - This could be the best CQB weapon in the game! Once you get the hang of the 4 round burst, you can put up quad feeds. My secret is a close by TI for immediate Revenge medals, and time to reload...