It is very difficult to give an exact answer to the question which renewable energy source is the most important. This is because we can judge each renewable energy source by different criteria, and in each case have completely different outcome. For instance if we look at the current level of development and price tag then wind energy would be the most important energy source, if we look at the potential of each renewable energy source then solar energy would be the most important energy source, and if we look at the availability to use it in every corner of the world then biomass would be probably considered the most important energy source.
Still, most energy experts will tell you that two of the most important renewable energy sources are wind and solar energy. This is because wind currently leads the way as the most developed energy source, and solar energy is expected to dominate global energy market in years to come.
We must not also forget that fossil fuels (coal, oil, and natural gas) still play the most important role in global energy supply, and they look very likely to remain dominant sources of energy for at least next couple of decades because renewable energy sources still need to significantly decrease their costs, find adequate storage solutions to solve intermittency issue, etc.
The importance of renewable energy sources is multi-dimensional, not only are they expected to cover for fossil fuels once we run out of oil and natural gas, but they also provide energy security (by diversifying domestic energy portfolio), and also energy independence (by reducing need for foreign fuel import). Their environmental dimension is well known as they are clean energy sources that do not release vast amounts of greenhouse gases that cause global warming like this is the case with fossil fuels.
It is of vital importance to further develop renewable energy technologies in order to not only make renewable energy more cost-competitive with fossil fuels but also much more efficient. Maximum efficiency and low costs are the two main goals that renewable energy needs to achieve in order to end the dominance of fossil fuels.
Labeling just one renewable energy source as the most important would be definitely wrong thing to do because each country determines importance of certain renewable energy source. For instance wind energy could be considered as the most important renewable energy source for United States, geothermal energy for Iceland, solar energy for Spain, tidal and wave power for Britain.