Sorry, I have to say either Nihilus or Revan, for most powerful. In a fight Nihilus would win, of course, because he would just suck the life energy and power out of him like a mosquito in a juice box, but Revan, as I am told, was so powerful that he was able to use the Force to live on hundreds of years after the events of the Old Republic. Not sure if that's true or not, but here's something else: Revan wasn't just powerful, he was influential. Without Battle Meditation, he inspired half of the Jedi Order to follow him into the Mandalorian Wars. The only reason he was captured by the Jedi in the first place was because Malak turned on him and fired on Revan's ship in a complete blindside. And even then, he was so powerful that they couldn't just capture him, they had to wipe his memory too, to prevent him from escaping and slaughtering the Jedi.
For most evil, probably Darth Sidious. The man waited in the shadows like a Spider, just waiting for the right moment to crush the Jedi Order.
For scariest, freaking Darth Sion, man.