I can confirm the 15 Grain one I recommended gets moldy. Found out the hard way.
In terms of widely available mass-market sliced bread my preference is for Oroweat Country Potato.
https://www.oroweat.com/products/sliced-breads/country/potato-0
Regarding the Asian-store white bread recommendation above I agree wholeheartedly that it produces the best toast you’ll ever get out of sliced white bread. But I’m not super keen on it for sandwiches (unless it is a toasted sandwich). A little too doughy for my preferences.
I don’t know if this is a national brand, but I like Nature’s Own Honey Wheat. It’s all I buy.
Hartford Farms whole grain nutty oat is my favorite. It’s nice and firm, holds up well and is yummy! There are actually pieces of nut in it.
I have some in my freezer waiting for when I need it again. It isn’t quite as good as the rye bread made in my neighborhood bakery when I was growing up, but it is close enough for mass market.
I used to work near an Orowheat bakery with an associated thrift store, which sold all kinds of bread and English muffins and Entemann’s too. They had a card that when you accumulated $30 of purchases they practically threw the contents of the shelves for you for free.
Orowheat has about the best variety of good whole wheat and whole grain bread too.
Roman Meal is good, but the slice is too small for my sandwiches, and too big for me to want to make two.
I love Arnold breads - 12 Grain, Oatnut, Potato, Italian…also sold under the name Oroweat, which seems to be highly recommended here.