I heartily agree about Vietnamese subs (meatball is my personal favorite) and Mother’s in New Orleans, especially roast beef.
One surprisingly good and little-known in the US variety is the bocata de tortilla espanola. This is a Spanish potato omelette, made with good quality eggs and potatoes deep-fried in good quality olive oil, on Spanish bread, which is similar to what we in the US call French bread but different in a way I can’t quite put my finger on. Sure, you think a potato sandwich would be bland as heck, but it’s not. I hope I get to eat at least one more good one of those before I die.