I don't think I could narrow it down to just one. Some faves of mine (in no particular order);
steak & cheese (or steak bomb)
meatball sub
ham & swiss on rye
good Italian sub
PB&J (J in my case being raspberry preserves)
grilled tuna & cheese on rye
No mayonnaise EVER, mustard where appropriate (kinda nasty on the PBJ :thumbdown

Other than the grilled tuna, never toasted.
Tough to find good sub rolls anymore.
Meatloaf and mustard on flax seed bread
I like meatloaf, but not in a sandwich. Dunno why, not much different from a hamburger.
I travel a lot and I actually like the Jimmie Johns chain for sandwiches. They're just a no-nonsense sandwich but have a lot of flavor compared to many of the other sandwich/sub chains.
Never heard of them, we don't have many of the chains here like in some parts of the country. We do have Subway, which is kind of "meh". D'Angelo's was good (loved their #9!), but I think they've gone out of business. Unfortunately, the mom & pop sub shops have kind of died out too.