Is fusion power still considered to be the most powerful energy source possibly achievable of technology by mankind, or there is any other type of fuel that has passed it? I'm not looking for anything outrageous like a hurricane engine or a black hole engine, but some kind of reaction scientists speculate may be a good source of energy for a power-plant or engine that mankind could realistically use if everything worked out. For example, horsepower yields to coal steam engines yields to gas engines yields to nuclear fission power yields to fusion power (not yet, but the next thing) yields to (fill in the blank) maybe later with better technology. If fusion is still king of achievable high power and there is nothing foreseeable to supercede it right now, that is fine, too. This is mostly a status check.