Fanless systems are the most electrical efficient. These are usually radiation based. Instead of heating the air and moving the air, the radiation directly heats the objects in front of it. Less energy is needed because less mass/matter is 'heated' and subtract the motor. With fan systems you need to heat the air first then move it, then the surrounding area heats up. Physics 101 .

I actually use a hybrid set up. I have a tank mounted radiant propane heater (dual head). Works well in the immediate area but doesnt put out enough energy to heat my non insulated garage. I have a small fan behind it the blows the rising hot air into the general area. If I wanted to heat my entire non insulated garage I would need a 60,000 btu heater... thats getting up in cost in equipment and fuel []

What is the most efficient in terms of money... depends on the total system efficiency and the cost of fuel/power. Use the energy calculator I posted earlier.

What is the most efficient in terms of energy (as in joules)... not electrical for sure! Natural Gas has the highest [link=http://en.wikipedia.org/wiki/Energy_density]energy density[/link] but the heaters are not 100% efficient...