Blankets which are thick, like cotton fleece blankets and wool blankets are the warmest. The fibers of wool are disposed to trap the air in between and retain it.   This mechanism works to keep you warmer throughout freezing nights in the tent. Fleece is also a good insulator, and synthetic blends used to create simulated wool do compete with pure wool standards.   But when it comes to protecting you from the chilly winds of frozen heights, nothing can stand up to the insulation capacity of pure wool.