Currently, the most efficient alternative energy source at around 95% in the case of large hydropower dams, and 90% for smaller ones, tied with tidal power (not widely used).
There are different types of energy that are considered "alternative". The details depend on the type of energy.
fossil energy alternative energy nuclear power
Anthracite has the highest energy
Alternative
If efficiency is 100%, each Joule of electrical energy is converted into one Joule of another type of energy.
nuclear energysolar energywind energygeothermal energybiomass energyhydroelectric energy
Efficiency is output power divided by input power. You somehow have to measure, calculate, or estimate both; the details will vary depending on the type of energy input and energy output.
Gamma rays.
It describes how good an engine is at turning fuel into the desired form of energy. High fuel efficiency means that as much as possible of the energy in the fuel has turned into whatever other type of energy that was intended. Low fuel efficiency means there's a lot lost in the transition.
A motor is used to power a windmill.
Gamma radiation is an extremely high frequency EM wave and is probably the common lightwave with the highest energy. Since energy is proportional to frequency, the highest energy wave would be that with the highest frequency.
Fatty and sugary foodstuffs have the highest energy content per weight unit.