I don't normally like sandwiches, but Pret a Manger has managed to make many of their sandwiches very tasty. I think they manage to balance the right flavors. Off the top of my head e.g. pickled red onions for sour, cheese/meat for salty, roasted tomatoes and chutney for sweet.

Ingredients they use aren't necessarily fancy or expensive, but they balance them appropriately and consistently.