It's going to depend upon local conditions. I'd have said a debri hut years ago but I've been hunting in places where that wouldn't have been an option. In winter here in northwestern Montana I'd have pine trees and snow for material. I've also gotten under the branches of a big fir tree and built a small fire to warm up when it was raining and snowing. It was quick, easy and comfortable. Lean-to's work well here. My favorite is a modified "A" frame with lots of pine boughs over it to shed rain/snow and a fire in front. I've built them using limbs from blown-down trees but in a true survival situation I wouldn't be so particular. I believe that there is no "one best" survival shelter but that a person should have several types in mind and use the one that best fits the local conditions.

The toughest place I ever saw to come up with an adequate shelter was in eastern Montana. There was nothing bigger than sage brush, no snow and hurricane force winds right out of the north (well, maybe not that severe!). The temperature was about -10 (f.) not counting the wind chill. The ground was frozen rock hard. If you couldn't find a draw to get out of the wind I don't know how a person would survive an emergency there.

Using a dome shaped shelter utilizes good airflow. Don't lay directly on the ground, whatever you do.

I agree with the debris huts. I am currently a student at Northern Michigan University. Albeit, Marquette is a small town, It is too big for me. Whenever there is a free weekend, day that I don't have to be up early in the morning, I take off to my debris hut in the woods of the outskirt of the town. It stops the rain and helps with warmth. I couldn't ask for a more peaceful relent from the town. In the winter though, I am going to have to go with a quinzee.

I have slept in debris shelters and lean-tos. I find that any shelter lined with one of those small reflective mylar emergency blankets will block wind, help shed water, and most importantly reflect the fire's heat. In novels and movies the lost kid always finds a handy cave. "Danger Will Robinson" Building a fire under some rock over hangs can cause that overhanging rock to break loose and drop on the fire builder. What ever you build cover the floor with pine boughs, bracken, etc. to keep yourself off the cold ground or you will never stay warm.