First of all, a perfect energy source should be extremely abundant source of energy, something like for instance our Sun.
Second, a perfect energy source shouldn’t be connected with any costs, it should be a totally free source of energy, or in the worst case connected with negligible costs.
A perfect energy source would also have to be 100% environmentally friendly source of energy.
It would also have to be renewable (naturally replenished) because finite energy resources do not guarantee 100% energy security.
It would also have to be 100% reliable, with 24-7 delivery, regardless of weather conditions, location, time of the day, etc.
It would have to be easily incorporated into our existing energy grid, and not require major technological advances and advanced scientific research.
It would have to be either available worldwide, or extremely easily transferred from one country to other.
It would have to be 100% efficient, meaning that there is no loss of energy whatsoever.
These are the major qualities certain energy source should have in order to be labeled as perfect. Neither renewable energy sources (on current level of development) nor fossil fuels have all of these qualities. Much is expected of renewable energy sources in years to come, and there are some energy experts who believe that solar energy might become a perfect energy source in years to come.