I've rather liked kabocha for a dry squash. It is rather dryer and denser in grain for the flesh than say an acorn squash.

Cushaw to my understanding and experience has a texture a bit more suited for pie making than mashed potato- it has a fine dense grain sort of like a step between kabocha and butternut. It's a bit less sweet than butternut IMO.
Banana squashes are good too.

When looking for seeds for dry fleshed squashes, note kinds that are are known for being long keepers. I've found in general the seriously long keeping squashes usually are the ones that are the best for potato substitutes.

Concur: hubbard types, Buttercup types, kabocha types tend to be the driest and best keepers although many of the C. moschata will compete. If you want an acorn size with hubbard taste and keeping qualities try Gold Nugget. Butternut types are IMO the best sweet potato substitutes.As for stringiness, newer introductions tend to be less stringy than the antiques.

I have not grown many varieties of squash, usually Kobacha or buttercup. Have lately grown Red Curry and found it to be very dry but tasty. Not stringy at all. Now one of my favorite to grow.

All the above mentioned winter squash are the species Cucurbita maxima. They have the fat stem. The maximas are the dry string free species. The butternuts belong to species C. moschata. And the acorns, delicatas, jack-o-lanterns, zuchinis, and most summer squash belong to the species C. pepo. The pepos tend to be string and wetter. E.g. spaghetti squash is a C. pepo.