I don't see Hannibal as a villain either, although there's something not quite right about him if he makes dogs back away and cringe in his presense. I'd have to say Palpatine is the all-time "best" villain simply due to his irredeemable evil. Darth Vader redeemed himself. Palpatine is just pure evil - the antichrist of the Saga.
That's about the best analogy there is to Palpatine. Darth Vader was powerful and used his power for evil intent. Even when he was Anakin, he gave in to vanity and conceit. But still, he was a minion, not unlike Maul or any other Sith apprentice. Palpatine, on the other hand, carries on the dark legacy of Darth Bane, covertly tears the Republic apart, and instates the most tyrannically totalitarian galaxy-spanning Empire of all time. But that same Empire was short-lived compared to the Republic that it replaced, which later replaced it in turn with the New Republic.
It makes me harken back to the history of the American Revolution, the creation of the American Republic, and all the Evil Empires that have been defeated by us and by our Alliance with other worldwide freedom fighters. Bolsheviks, Nazis, Fascists, and fundamentalist false prophets have been layed waste to by those who would rather die free than live oppressed.
Palpatine is the antithesis of that. He's the ultimate oppressor - the ultimate evil. He revels in his own power and marvels at himself. He's the definitive Beast. He has a cruel heart, a cruel mind, and a total absense of conscience. He's also cunning, covert, and patient. He's no ordinary villain. He's no jackbooted lacky like Darth Maul. He's no common street thug like Sebulba. He's pure evil manifested in the flesh. He's the very essence of what government is all about.