Stainless steel is considered the best material for thermoses. Stainless steel does not transfer as much heat as other metals making it a good insulator. It is also resistant to acidic drinks and has a neutral effect on odors and flavors. Vacuum-sealed stainless steel thermoses have two layers of stainless steel. These are the best when it comes to insulation and heat retention for your coffee. Thermoses with a stainless steel inner layer and a plastic outer layer will not last as long and are prone to breaking.