This is a very subjective question. Power can be defined in many ways. One could certainly argue that humans have the most power to globally effect life on earth, as well as non-living things. However, one could also argue that viruses can evolve faster than humans can ever hope to and can wipe out most, or perhaps even all, humans, given the right mutations.
As the previous answerer mentioned, dinosaurs could be considered the most powerful. They were certainly very large and likely very strong, which is one measure of power.
One could also measure power as to how much biomass that species has on earth. I believe antarctic krill, Euphausia superba take the crown there.