I was in grad school researching in this area and I can tell you this...
....The absolute best solution is solar power. There are other great ideas out there (geothermal, hydroelectric, biofuels, nuclear) but they all suffer from one very limiting thing, even if we exploited those technologies to their absolute fullest they would not be able to provide the amount of energy our world uses.
Solar, however can. There is enough potential solar energy that even with a low efficiency of about 10% we could provide over 100x the worlds current power consumption. The biggest problem with solar right now is cost. Until quite recently solar power cost about 10-15x that of energy produced from sources like coal and oil. However, the cost has come down a ton and will hopefully be economically viable in the next 5 years and thus able to go mainstream.
Solar power biggest limitation is, well, night. With a bit of luck hydrogen fuel cells will also become viable in the next 5 years. The most promising plans have solar power being used to perform hydrolysis to provide the hydrogen for a fuel cell. The system would be completely sell contained, pollutionless, and as a bonus would be off grid (a substantial amount of the worlds power is used up in transportation losses).
and that's all i have to say about that. xD
