Stuffing wins if for no other reason than all the other gay dishes listed can be had year round.
I think it also wins because it's the dish that's least terrible when mailed in.
Mailed in mashed potatoes are worthless. You put enough gravy on it, fine*, but by themselves nothing special. *Even then, it's dependent upon someone making proper gravy, which is no guarantee.
I've never actually had green bean casserole that's been made with a lot of effort. I didn't grow up with the stuff and the only time I ever see it is when my wife's family makes it and, best I can tell, they just throw together some canned or frozen green beans with some cream of mushroom and dump some canned fried onions on top. I'm sure it could be decent, but I've been so turned off by the whole concept, I don't care to try.
And since I don't think fresh green beans are really an option in late-November, I'm not sure how possible a great GBC is. If it involves canned or frozen green beans, I'm not interested.
On the same lines, I'm not sure I've ever had a great sweet potato casserole, but the basic offering is very unspectacular. Usually, it's some crap out of a can loaded up with marshmallows. Don't recall too many fights over who gets the last bit of SP Casserole.
I'm not sure I've ever had Mac 'n Cheese on Thanksgiving, but if enough people have eaten that boxed crap, it's no wonder it's not getting more votes. Even a lot of homemade M&C is pretty underwhelming.
But stuffing.....while made from scratch with plenty of sausage is much preferred, the boxed crap can be surprisingly decent. Loaded with a ton of sodium to help the cause I'm sure, but if given the lazy options between all these, I'd take the stuffing.
And frankly, many T-Giving sides (and the meat itself) are generally pretty bland, even when homemade. There's a ton of it and people love them because they are traditional, but the most common are fairly bland dishes.
Actually, I think that's exactly why we
only eat them at Thanksgiving.
Except stuffing, which in nearly every form, is loaded with seasoning.