Size, color, design, and installation type will determine the best backyard door for your home. Doors generally come in two varieties: pre-hung and slab doors. A pre-hung door will already be hung in a frame when you buy it from the store, whereas a slab door will just be the door itself; it will have to be installed in an existing frame or one built specifically for that door. Pre-hung backyard door models are easier to install, since they basically just need to be raised into place, but they also offer less flexibility in size and design.
Think carefully about the materials used to build the backyard door. Some doors are made of wood, which can be a good material if the wood is treated to resist weather damage, or if it is a wood that is naturally resistant to water damage and bug infestation. Such woods tend to be more expensive than others, so many homeowners choose other materials such as plastics, fiberglass, or composite materials. These materials can be less expensive in some cases, and they will generally have a long life because they are resistant to weather damage. Some plastic doors can warp after long exposure to direct sunlight, however, and some lower-quality backyard door models may fade in direct sunlight as well.
It is best to choose a door that is thick enough to provide some insulation, especially if you live in a generally cold climate. Solid wood doors can retain heat, as can some metal and composite doors. Hollow doors should be avoided, as these will not retain heat as well as other designs. Hollow doors are, however, the least expensive option for a backyard door, and they can be used in conjunction with an outer screen door that can be fitted with storm windows, thereby adding more insulating properties.
Think about whether you want the backyard door to feature windows. Many doors will feature small windows that allow you to look into the yard clearly, while others may be solid doors with no windows at all. This is generally a stylistic preference, and either door will work just fine. Keep in mind that some heat can be lost through the windows, especially if the windows tend to be large or take up a lot of space on the face of the door. If insulation is a primary concern, a solid door with no windows may be best.