I think anakin had the most potential, he did have the highest midicloron (sp) count ever. But even at his height of power, I dont think he couldve taken on Revan, let alone the emperor.
I think I'd vote Darth Sidious, not only did he bring down the republic which stood for twenty thousand years, he also kept his true identity hidden from some of the greatest jedis to ever grace the universe. He also trained some of the most badass bad guys to ever grace the movie screen. As far as accomplishments go, its hard to beat his resume.
I dont see how Luke could be the most powerful, sure hes anakins son, but he wasnt conceived by midiclorons(sp) like his father. Also he never really got the full training of a jedi, and started really late. He has potential, but I dont think he ever reached the pinnacle of it. Just my opinion.