What is the most efficient fuel for cooking?

well, you can try to cook on a stone placed directly under the sun.. the time taken to cook will vary depending on the place you are trying it (in some places it might even take decades to completely cook).

Natural gas.    

If by efficient you mean provides the most heat for the lowest price then natural gas by far.

Every top chef in the world uses only gas.

It costs many times more to use electricity and it does a poor job at cooking

Under ideal conditions, solar. A parabolic reflector can boil water as quickly as a fossil-fuel-using stove and has no cost for fuel nor does it produce any pollution. Unfortunately, conditions are not always ideal.

If you restrict yourself to fossil fuels (or even biomass), the problem gets quite complicated. you have to consider both the costs (environmental as well as financial) of the fuel an the costs to clean up afterward. When I was heating my house with a wood stove (using wood saved from the landfill), I would sometimes have a pot on top of the stove. Place it correctly and it can act as a slow-cooker. Although the wood did put carbon dioxide into the atmosphere, the ash served to sweeten the soil on my garden (or as grit on my icy sidewalk), so there was no solid waste to dispose of. The landscapers who supplied me saved fuel by not having to haul the wood to the transfer station. I got my exercise without having to go to a gym. And the landfill lasted a wee bit longer by about 600 cubic feet per year, based on my average usage.

Induction is the most efficient.  It is also the cleanest.

I had a gas stove once and loved it. It was great for cooking stir fries in the wok as it had more heat around the base than an electric element can reach. Plus it heats faster.