"Greatest" or "Best" lists always come with a personal bias. What do you need the shotgun for? Hunting? Home Defense? Door Breaching? CQB? Clays? Personally, a lot of these "great" shotguns have absolutely no place in my safe. In fact, the two that I have aren't even on the list, and I don't regret the two that I have.
Whether or not they have a place in your safe or not depends on your needs and means, but isn't a quantifiable like "best" should be.
I'll use the example of the Boss and the Krieghoff K-80.
The Boss set the stage for almost all other O/U's that came afterwards. It was an incredibly unique design, brilliant in engineering, and has become the standard design that most manufacturers now follow.
The Krieghoff K-80 is another in a long line of brilliant designs by the late Crawford Loomis of Remington. It's the winningest competition shotgun in history, it can go literally millions of rounds, and if something does break or wear out, it's infinately rebuildable. You literally cannot wear one out.
There are some other guns on that list are mediocre at best, and knockoffs of other brands. There are some on that list that are there to appease collectors, even though they really aren't that great a gun.
Still others are there to appease the budget conscious, lest the writer be accused of elitism.