What is the most efficient renewable energy source?

This would probably be hydroelectricity. Such plants use the same principles as any other power plant to convert energy of motion into electricity. Solar, is only about 30% efficient, being that it runs on heat only. Wind is unreliable. The river flows ever on…

The ocean because the tides are constant and the waves are a bonus.

sun is most renewable source of energy.
sun continuous to exist even when earth is destroyed.

The sun doesn't run -just- on heat, it runs on light. However, the sun is only there during the day, which is a bummer. The cool thing about it is that it's free… and no pollution…

I don't know, there are several options that are certainly more environmentally friendly than what we are currently using, for instance, water, wind, sun, …

From the standpint of cost–hydroelectric power. that is, of course, limited–and already utilized to a great extent.

Of possible alternatives, probably solar–the initial cost is still fairly high–but the operating cost is nil.

Of alternative "portable" energy sources (i.e., for cars) probably ethanol.

Air movement. Easy to harness and inexpensive to implement.
Also very environment friendly.

Yes its Hydro electric, but it has been delisted as being environmentally friendly. It kills fish. I think geo-thermal is next up. Wind is third. Solar isn't worth mentioning. I'm measuring efficiency by cost of developping and maintaining the energy source. Every dollar you put in, you get this many watts. And in response to wind being unpredictable, so is solar. There might be clouds in the sky too. I like wind the best, since Geothermal is severly limited by sites. You could tell which is the most cost effective, by what percent of our energy comes from that pareticular source. By the way, we could also make nuclear plant renewable too. I think we can build feeder plants that continously regenerates transuranium. Its not very environmentally friendly.

if by efficient you mean the one with the highest power yield to our technological level, its hydro power. there's more force in moving water than wind, so water can turn very heavy turbines. hydro power can operate at all times of the day whatever the season or weather, which can affect wind and solar a lot.

Air pressure,established in a water column, harnased with the help of a negative atmosphere.