So far, the greenest way to generate energy is nuclear power. When the impact on the environment (in terms of mass vs. energy generated) is an inverse-squared relationship to the amount of energy generated (m = e / c**2), that's hard to beat when most other means of generating energy so far only achieve a direct energy-to-mass relationship.

The greenest way to generate power is from natural power generating sources, but these ways are not always the easiest ways. For instance, some green ways to generate power include the following:

1. Hydro electricity (from flowing waves of water)
2. Solar electricity (from the sun)
3. Wind electricity (from wind turbines)

I would think that solar is the easiest when you're in an area that has a lot of sun, but then you have to get solar panels, which may not make it as "green" as some may think, and it's expensive. Depending on how much wind or water movement, that will correlate to how much power you will generate.

So, if it's a very sunny day out, you can generate a lot of power, but if it's a cloudy day and it's raining, then solar electricity will not generate power as much.

I would think that hydro electricity would be the greenest, but then it has its drawbacks, such as killing fish or not being friendly to the sea life.

To answer your question we need to examine the environmental cost to generate electricity via the traditional methods and then compare them with alternatives such as Hydro, Solar (Photovoltaic) and Wind.

Each method traditional and alternative has some kind of environmental impact whether in the production or in the development to production. e.g. Wind the production process to melt the steel or to create composites to build the turbines plus transportation to site all has some kind of environmental impact. Same can be said for Hydro and Solar. However this is where they differ in the generation process, while fossil fuel negatively impacts the environment while being developed they also negatively impact the environment during production. Where as Solar, Wind and Hydro don't impact the environment during energy production, fossil fuel does due to heat and carbon and other gaseous emissions.

Which now bring us to the question of which is the greenest method of electricity production? In my opinion it is Solar, why solar? Solar has the least impact on the environment during the generation process and is the most direct to energy generation process available. Wind and Hydro still have to turn turbines and take up valuable real estate in most instances. Where as solar can be place on most roof tops and depending on it purpose in other convenient locations. Solar is currently poised to become the most ubiquitous free energy solution available especially when it comes to lighting. Check out Outdoor Solar lights or Solar Deck lights just to mention a few. Furthermore the technology is advancing to the point of what is called grid parity (read this article at Solar deck lights to learn more about grid parity and the development in the solar light industry).

With this in mind I strongly believe when we consider the environmental cost to productivity ratio Solar is the greenest.

Here's a funny and intersting answer for ya! Not trying to get best answer with this one, just thought of this and thought it was funny.

How about a gym that takes advantage of all the kinetic energy spent in their establishment by hooking electricity generating motors to all the equipment, like exercise bikes and treadmills, so when someone exercises they are producing energy for the building?

Air Turbines

What kind of power (electric, transportation, cooking, other)? How much of it do you want to generate (home, country, Earth, small job)? These are vital parameters needed to try to answer your question.