The best alternative is water and several tests have been made successfully to generate energy from the water. Although it is still controversial issue but here in Pakistan one of the engineer has run a car on water. It was shown in the presence of media but due to political issues and pressure from the big companies it was buried.
So I personally think water may be the best alternative for oil.
It's all about natural energy. Solar, wind, water, and possibly geothermal energy will play roles. I just pray we don't start using nuclear energy because it's works out good, but the risks are great. When I say great, I mean high.