oil
a friend of mine (joe) is fond of saying that oil is so valuable that our grandkids will be appalled that we burned it. he's right and wrong. oil is easily the most valuable stuff on earth. but our grandkids will be appalled that we did anything with it other than put it in the tank. in terms of energy per pound, nothing comes close to oil's products: gasoline, diesel, and aviation fuel. you can't fly a 747 on batteries. nor are we likely to. ever.