What about Exar Kuhn?
He was well 'ard in Jedi Accademy series, and made a fairly inexperienced apprentice into a sick sith - Kyp Duron.
As far as the Emperor goes, I have to say he's the winner. Sure, he's not as menacing as Vader, and he's not all over you like Maul. But remember that physical appearence isn't the most important factor (yoda, anyone?) During episode II and III Palpatine will probably go through 2 or 3 more apprentices, and he will use and manipulate them ruthlessly. This is one powerfull and power-mad dude.