Star Wars Episode III

[Carnivore]

Mace Windu could have killed the Emperor and would have had Anakin not intervened. Mace was the second most powerful Jedi Master.

Yoda could have killed the Emperor, but he was racing against the clock a bit. As soon as they started tearing up the Senate chambers, it was only a matter of time before the clone troopers started pouring in. Then the odds would be decidedly against him.