This is a difficult question. As far as film appearances, I can't say anything other than The Emperor (I generally prefer to call him by his title than by his full Sith name). However, there are several older and less-appreciated Sith that I really admire, such as Freedon Nadd (a character I wish to learn more about) and Ludo Kressh. But the one we know the most about that did not appear in a film, whom I could consider a favorite, is Wyyrlok III. Deeply underappreciated. I have a theory that Wyyrlok I is, in fact, Mas Amedda, and was taught by The Emperor some rudimentary Dark Side knowledge, leading to his offspring becoming the start of the great Lorekeepers of the Legacy Era. I think that is the best answer I can give.