Vietnamese iced coffee (Cà Phê Sữa Đá) is known for its sweetness and complexity from its one secret ingredient: sweetened condensed milk.
This recipe recommends using a dark roast coffee with a smoky characteristic (the author uses Hayes Valley Espresso from Blue Bottle).
Try to use a brand of condensed milk from an Asian supermarket (e.g. Longevity Sweetened Condensed Milk).