...is Antimatter. All we need is about 3 tonnes of antimatter and a suitable power station to collect the energy, and we could generate more power than mankind uses across the entire planet, including electricity, cars, planes, the lot.
To put this in perspective, 3 tonnes of antimatter should fit comfortably in a small car.
Unfortunately at the moment the cost is around a billion trillion dollars. This would take around 15 million years for the world to pay for even if we made nothing else and didn't need to eat.
But let's be optimistic. Like solar power, the price is sure to come down.