As in camping tents the best backpacking tent depends on the type of camping and the amount of people that are going to use the tent. If just you are going to sleep in a tent you do not have to carry a tent designed to sleep 3. Neither do you want to carry a four season tent in the summer.
When you are backpacking, weight is very important and quickly adds up in your pack. Your goal is to carry only necessary items. Summer tents are the lightest because you do not need any insulating factors and the flies are smallest.
Winter or four season tents use heavier fabric to withstand higher winds and weight of snow. Four season tents also have heavier frames or poles to support a snow load. The heaviest backpacking tents are the expedition tents that are designed for the worst weather possible. The flies have webbing that goes under the tent to keep the fly in place and help to prevent the wind from getting under the fly not to mention the numerous tie downs.
As I said above, you only want to carry what you need. A summer tent does not have a full fly, so they do not have vestibules. Vestibules are a handy place to put packs and gear to keep out of the weather. More room means more fabric and larger poles to support the fabric, so you do not want to carry more room than you need.
Naturally you carry more and bulkier clothing in the winter to keep warm. Therefore, winter tents have more room per person. Weight and room go hand in hand: the more room a tent has the more it will weigh.
Summer tents are the lightest because of the screen to fabric ratio and the size of the flies As in the page about camping tents, there are various types of poles.
While strength is not a major factor because of the lightweight material used in summer tents, your poles should be able to withstand normal use. Fiberglass poles can fray and splinter with age and stress but can be temporarily repaired with duct tape or making a splint from branches. Aluminum poles are lighter but cannot be repaired as easily in the field.
Summer tents are only good for the summer. Cooler nights can cause moisture from perspiration and breath to collect on the inner tent walls causing puddles inside the tent. The flies do not go to the ground so they offer more ventilation.
A good three season tent will have a full fly that reaches the ground and a high screen to fabric ratio. The more screen the better the ventilation will be. The fly acts as a dehumidifier on cool nights with the inside moisture collecting on the inside of the fly instead of the tent. That is why it is important to have a lot of screen area. The fly acts as the thermal break, not the tent.
While tents are made of water repellent material the fly should be waterproof. You will be able to feel a slight coating on the fly. Also, make sure the seams are folded over and double stitched. Though the fly acts as insulation, on hot summer nights you can have the fly open or removed to allow fresh air into the tent. The fly should also have a vestibule for gear storage.
As in all good tents the floor will be constructed of a heavy coated nylon coming up the sides of the tent a few inches to form a tub. The poles should be coated aluminum because to equal the same strength fiberglass has to be thicker and weigh more. Also, fiberglass looses it's flexibility as the temperature drops.
During the winter is when you will encounter the harshest weather. Your tent has to be able to protect you from this weather. Besides having the same floor and screen design as 3 season tents a 4 season tent has to be able to support a snow load. This means the fly fabric and poles will be heavier and there will be more poles. The fly is also designed to allow the snow to slide off lessening the weight of the snow.
Because you take more and bulkier clothes with you during winter hikes your clothes will take up more room. Designers take this into consideration and make 4 season tents a little larger to accommodate the bulky outerwear even though a smaller tent is easier to heat.
4 season tents designed for extreme weather will have numerous tie downs on a heavier fly and webbing that runs under the tent as in the picture at the top of this page. The fly should have extra material along the ground contact so snow or dirt can be piled to seal the extra material to the ground preventing wind from blowing into the tent. The poles may be made of exotic materials such as titanium or carbon alloys.
To find the best backpacking tent you have to figure out when you will be backpacking, the type of weather you will run into and how many people will be sleeping in the tent. If you backpack during different seasons you should invest in a tent for each season. While you can use a 4 season tent in the summer a 3 season or summer tent would be more comfortable and you can save the more expensive 4 season tent for when you need it.