Generally, every safe goes through a rigorous testing process and the manufacturer will use all the means necessary to try and break the safe, before releasing the safe on the market. I mostly recommend steel as it is one of the most durable and highly tested materials on earth. Steel has been proven time and again to withstand high amounts of heat and water. Another great thing about steel is the ability conduct electricity and keep the charge in the steel frame. This will ensure that your firearm does not go off or that any items are damaged, should intruders try and use electricity to pry open your safe. Even though steel is not 100% impenetrable, I highly recommend paying a little extra and choosing a steel safe.