How powerful was Mace Windu?
Not many people have realised that Mace Windu was a more powerful Jedi that Yoda. How so? Yoda was a bad ass - we saw him take on Count Dooku and the Emperor!
And that is true, Yoda fought Dooku in Attack of the Clones and fought an epic saber duel against Palpatine himself. But Yoda did not beat Palpatine.
Who did beat the Palpatine? Mace Windu did. He beat him fear and square. It was only the naive intervention of Anakin Skywalker that allowed the Emperor a literal window of opportunity to send Mace out the window to his presumed death.
I'd thus contend that by virtue of Yoda being unable to defeat the Sith Lord and that Mace managed to defeat him, Mace Windu was a more powerful Jedi than Yoda.
It's arguable of course, there are many other attributes of being a Jedi that can be defined as powerful - however if you want to boil it down, Jedi are knights and how are knights ultimately judged? By their swordsman ship.
Which raises an interesting point. In the novelisation of Revenge of the Sith, Windu suggests to Kenobi that Kenobi is the better swordsman, thus making him the most powerful Jedi!