You might already have a heater, but a portable propane heater is going to be the most efficient. It uses less propane, and no battery draw to operate.
Some of these heaters use small 1 lb propane tanks, but you can get a hose to hook it up directly to your larger propane tank if you have one. Be sure to keep a window or two cracked to let in oxygen and reduce condensation.
BTUs are a little complicated, but all you need to know is the more BTUs, the more capable the heater is of heating a larger room. 3,000 BTU is good for 100 sq ft and 8,000 BTU for 230 sq ft
Mr. Buddy heaters require higher-pressure propane into the unit if you're using a hose, and they regulate the pressure down at the unit. Olympian Waves use low pressure propane. The difference is mostly just you use a different hose, but for the high pressure variants, there is more pressure on the hose. Both types are safe, but Olympian Waves tend to be a little safer.