If I had to pick a shotgun that I had to train folks to use(and I have), it would be a Mossberg of some variety. Meets requirements for everything including cost. Going up it would be a Browning BPS, 870, in that order. If they still made them Ithaca would be in the mix.
Best means many things to many folks. I never liked Glock pistols till I had to train people to use them and did not speak the language, simple, simple, simple. And simple is good. Same with shotguns, if a Mossberg had downward ejection, it might be perfect-no wait-that is a BPS.
Now the above is based on the ease of training/use for everyman. For myself, I would probably stick to the 1100 substantially modified with extended mag, trigger job, rifle sights, and a ported barrel. Softest shooting auto going IMHO. An unmodified Benelli M2 or Beretta 1201 would fill the bill while kicking my butt somewhat more.