I noticed a lot of people rated the Emperor high up- I think he was more cunning and manipulative than he was powerful. He got his rear handed to him by Windu, and as for the fight with Yoda, I don't what Yoda's deal was- he should've done a lot better against Palpatine. If Yoda had been a hundred years younger, Palpatine would not have stood a chance. Notice once Vader came on the scene, Palpy made him do everything! Clearly Anakin Skywalker was the most powerful Jedi ever until he lost his limbs, and Palpy knew it. Yeah, Anakin got beat by Dooku, but that's because he was young and stupid. Dooku was more powerful than he let on- he was more or less betrayed. If he had known he was fighting for his life in ROTS, he would have fought much differently.
I agree with the statement that was made earlier about Obi Wan being smarter than he was skilled. In pretty much every major battle he was in he was bested by his opponent and used his head to win (he was "almost beat" by Maul, Jango Fett(!), Dooku, Grievous, and Anakin- and in all those cases, pulled a maneuver out of his ***, or took advantage of an opportunity to come out on top). So, considering all that, here is my ranking:
1) young Anakin, by a long shot
2) Mace Windu and young(er) Yoda, tied
3) post-op Vader. even without flesh limbs and force lightning, his command of the force was frightening
4) Count Dooku- he could have ruled the universe, but he trusted Palatine, which was his fatal mistake
5) The Emperor. We must not underestimate the power of Palpy's cunning. He tops the whole list where that is concerned.
6) Young Obi Wan and Darth Maul, tied.
7) Qui-Gon
8) Luke Skywalker. His greatest powers were hope and love, but Darth Maul would've shredded him. Granted, he had a fraction of the official training then the rest of them did, and he did really well with what he knew.