Virtually any cut can be used. I think the larger the cut the easier to slice. With mashed potatoes and gravy, a more marbled and flavorful cut might be in order. A Rib Roast (I would not put gravy on this), a Top Sirloin Petite Roast, Tri-tip or a Sirloin Tip Roast would be good and flavorful. For a sandwich that say might be open faced with gravy on it, or a roast beef sandwich which might have mayo, mustard and cheese to add flavor, maybe a lesser cut like an Eye of Round cut very thin so it is very tender!