Energy density is a very important concept, one that could create a new technological revolution. In the present time several storage elements retain the highest energy density: first is nuclear energy and after hydrogen, gasoline, propane, fat and a long list of others.
Gasoline is then in the top ones, explaining why that insane quantity of combustion vehicles are able to transport as much weight, or why the planes can fly with several models over the five hundred tons mark.
Hydrogen must be very powerful because the stars use it in their constant production of energy and some are thinking that could be a substitute for gasoline in a near future. It presents some challenges, but you need a very good reason to forget a star —the Sun— developing energy for powering in theory more than one billion human civilizations.
There's a known problem about energy density because current battery technology can't compare with other energy densities. Batteries based principally on Lithium-ion are able of reaching less than 1MJ/kg, while gasoline is more than 40MJ/kg meaning a 1:40 ratio. If some battery could reach the same energy density multiply then its lifetime by forty, and that's like a dream for a big quantity of people.