Choosing the best waterproof luggage starts with determining what you will be using the luggage for. In general, luggage is stored and protected from the elements during travel, so if you are traveling by plane or train, a water-resistant piece of luggage may be sufficient. If, however, you will be carrying your belongings through inclement weather, you will need waterproof luggage that is not only resistant to water damage, but also lightweight, easily transported, and durable. If you are transporting electronics, consider hard shell luggage that will be waterproof and durable.
Duffel bags are the most common type of waterproof luggage. These duffels occasionally feature shoulder straps so the duffel bag can be worn as a backpack, or at the very least a shoulder strap so the bag can be carried on the shoulder. Consider this style of bag if you will be toting the waterproof luggage over longer distances, say on a hiking trip or walking through a large city. Duffel bags can come in a variety of sizes, so you can feasibly fit a significant amount of belongings in this type of luggage. Make sure the bag features taped seams; moisture is most likely to penetrate a bag at its seams, so taped seams will prevent water from coming in contact with your belongings.
If you prefer waterproof luggage that more closely resembles more common rolling luggage, consider hardshell polycarbonate or plastic luggage. These units are convenient, durable, and waterproof, depending on how the zippers and seams are treated. The polycarbonate shell makes the luggage useful for transporting fragile items such as electronics, and the shell is waterproof by nature. Such luggage may feature zippers, however, and these zippers will need to feature some sort of waterproof system to ensure water stays out. If the bags are hinged, the lip of the two sides of the bag will need to feature a rubber gasket that will keep out moisture when the bag is closed tightly.
Dry bags are commonly used by river runners and boaters to protect items that cannot get wet. Some dry bags can be used as waterproof luggage, but if you choose to use such bags, you will need to make sure straps and buckles can be tucked away during transport on airplanes. Airlines use conveyor belt systems that can catch straps and buckles. Some dry bags are transparent, too, and you will want to avoid such bags when traveling to prevent potential thieves from being tempted by the contents of your bag.