Forget Vader - Senator/Chancellor/Emperor Palpatine is the true villain of the saga. It is he who meticulously plans the events that allow a humble Senator from Naboo to become first Supreme Chancellor of the known universe and, ultimately, unimpeachable Emperor.
It is he who orchestrates the Clone Wars and the whittling away of the Jedi, before the hammer-blow of Order 66 and the final cull. And, if you want some hyperbole (the Emperor loves hyperbole), it is he who is the most evil character in movie history.
Yet it's entirely possible that Palpatine would have been just another power-crazed bad guy had it not been for Lucas' decision to replace Clive Revill, who voiced the role in the original version of The Empire Strikes Back (the face was provided by an elderly woman; and the eyes by, of all things, a chimp), with Scottish actor Ian McDiarmid for Return Of The Jedi.
McDiarmid injected the character - whom Lucas modelled on various despots, including Hitler - with elements of Othello's Iago, creating a coldly calculating character who persuades everyone of his noble intentions while working to destroy them. His fake bonhomie and subtle insincerity renders his rise to power entirely plausible - the Darth Plagueis speech in Episode III is possibly the finest piece of acting in the entire franchise.
But when the veil drops away, and McDiarmid gets to cut loose as Darth Sidious in Revenge Of The Sith and the Emperor in Jedi, subtlety takes an early bath. Snarling and sneering for all he's worth, helped by a generous smattering of the best lines, McDiarmid's turn as the Emperor is a go-for-broke depiction of pure malevolence that has deservedly become something of a cultural icon.
In the Expanded Universe, Palpatine has been brought back to life a number of times, via some handy clone bodies - an honour afforded no-one else, not even Vader. But then again, when you're so evil that Darth Vader is your bitch, perhaps special treatment is in order.