I think, I wouldn’t be very original if I say that the image of a villain that has impressed me more than the majority of other literary images of this kind is the infamous Lord Vetinari from Pratchett’s Discworld series, even though he does not technically belong to that category. Nevertheless, he can hardly be called a positive character.
Being the ruler of the Discworld’s main city, Ankh-Morpork and, in fact, possessing absolute power, he uses it only for the greater good of the city or, at least, for this good as he understands it. While being actually a dictator and exercising a purely pragmatic set of views unlike that of Machiavelli’s (in fact, his name is a sort of pun inspired by the family name “Medici”) and believing that in order to keep the power, the ruler can resort to any means, he rarely uses his power for personal benefit, being, in fact, rather ascetic.
In following one of his major principles, “don’t fix what isn’t broken”, he does everything in order to provide maximum stability of the system, without, however, applying any violent means of doing so unless they are absolutely necessary. According to him, there can be no change for the better – only the absence of the change for worse.
In Lord Vetinari Pratchett created a villain that is both appealing (as villains often are) and reasonable. Although everybody understands that it is not exactly a nice person, the reader cannot but admire his mind, ability and the fact that he remains to be the ruler almost for the entirety of the series, always managing to put things back to normal when everything starts to fall apart.