Hawashi is my favorite for how simple it is to make and how easily you can store the filling and then make one quickly at any time you want (picrel).
German Krabbenbrötchen (crab sandwich) but I never make that myself, gotta get it at the local market.
>Honorable mentions
Philly Cheesesteak
Cubano (Must be a good one though, they are relatively easy to fuck up)
Bahn mi
Arepas, though highly dependant on the filling
If we consider Döner a sandwich, then döner.
Mettbrötchen for nostalgia's sake, though that rarely comes with a top lid, so it doesn't really count.