I suspect Alamosaurus, since it's the only North American sauropod of the Late Cretaceous known from good remains.
But I doubt they ranged that far north. Maisauria is from Montana. Alamosaurus was in Texas, New Mexico, and Utah which was its farthest northern extent. It doesn't belong in the more swampy habitats of Campanian-age Montana (no sauropod does), but nevertheless this is a very beautiful painting.
In the earlier Cenomanian epoch, Montana wasn't so swampy, and there were some sauropods like Sauroposeidon (juvenile remains now at Yale Peabody Museum) have been found from those older rock layers.