This is a difficult one. Voldemort is supposed to be very powerful, and held his own against Dumbledore. On the other hand, Yoda is old, powerful, and can bounce around like a pinball on speed. In the end, I'd give this to Voldemort- all it would take is one well-placed killing curse, while there is no gauntee that he would stay dead if Yoda killed him.
Another close one. I'd give it to Dumbledore, as it would seem he is more powerful than Palapatine (or at least holds more tricks up his sleeve), but his hesitation to kill Palpatine would lead to the Emperor frying him like MAce. Winner: The Emperor.
And I'm[?b] a damned loser...