"Most powerful" is all a matter of perspective, and in many respects does not exist.
Mace Windu is widely regarded as the most lethal lightsaber duelist the Order ever produced. Is he the "most powerful?"
Yoda is widely regarded as the wisest and most educated of the Order. Does that make him the "most powerful?"
Anakin Skywalker was the Chosen One, destined to bring balance to the Force. Does that make him the "most powerful?"
Obi-Wan Kenobi was one of the Council's leading Masters and the only one deemed able to defeat General Grievous. Does that make him the "most powerful?"
Qui-Gon Jinn transcended upon death and became one with the Force, yet still retained identity and consciousness, and passed this technique on to Yoda and Obi-Wan. Does that make him the "most powerful?"
The Sith fail because they buy into the concept of "most powerful," always seeking more and more power for themselves and the cost of pretty much everything else. But ultimately, that power is false, empty useless, and will probably only get them killed.
Power is not the end goal of the Force. Knowledge is.
Which Yoda had, he had 900 years worth of knowledge and was a swordmaster. Its been stated in the ROTS novelization(think that it was that book), that Yoda was the most powerful jedi during that time but not even he could beat Sidious.