Darth Vader is possibly the only villain with a presence so big that he's been the subject of six blockbuster movies.
Lord Vader has royalty written all over him. The samurai helmet, the breath mask, the opaque black lenses, the cape, all in black: it's immediately obvious that this is a villain. His unnaturally regular breathing is the loudest thing about him. When he speaks, James Earl Jones' calm baritone gives this character an authority that is second to none on the silver screen. Vader is composed, regal, and utterly ruthless.
But his physical presence is not the only reason that Darth Vader has always been the Star Wars franchise's most popular character. The Star Wars saga is actually Darth Vader's story. I'm not just talking about the prequels, either (although I will get to them): the revelation of Darth Vader's past and the continuance of his character arc to its end is the engine upon which Star Wars is built.
Star Wars is the story of a great warrior who has succumbed to evil finding a measure of redemption.
Luke Skywalker, Vader's heir and the hero of the original trilogy, plots a similar course, but his arc is far less dramatic. In the end, Luke's story serves mainly to explain what happened to his father, and to provoke the final act of Vader's life.
Within the original films, Vader occupies a curious place. In the first Star Wars movie it's unclear what his place is within the Empire. He's obviously an important figure, but he does not have a formal place in the Empire's military. He is referred to as 'Lord Vader', but there is no evidence that the Empire has a feudal hierarchy (although some worlds, such as Alderaan, with its Princess Leia, appear to sport one). Vader's beliefs and his powers are scoffed at by the military officers, many of whom consider him a religious lunatic. When provoked into the infamous vulgar display of power in the board room, he then accepts General Tarkin's order to restrain himself, although it seems clear that he need not. Vader appears to be in charge of important tasks, such as questioning prisoners, and the generals consult with him often, but he is not a military man. He flies a fighter craft in a space battle, but he is not attached to a squadron and seems free to do as he pleases in an actual conflict. We are not shown his master, the Emperor Palpatine, until the following movie, Empire Strikes Back.
Empire Strikes Back is sure the strongest and most popular of the Star Wars movies. This, I think, is because it brings Vader into the foreground, even offering a glimpse beneath the mask. As the title indicates, in this movie Vader is given the initiative by his master and makes a show of how competent he is. His victory is assured despite the rebels' best plans.
The Star Wars prequels reinforce all of this to the point of redundancy. All through the prequels we hear about how Anakin Skywaker is the chosen one, Anakin is the messiah, Anakin is the one who will save the galaxy--but the old Republic doesn't really seem to need saving. In fact, it seems to be in pretty good shape; Anakin's rise in fact leads directly to its destruction.
The Jedi council claims that Anakin will bring 'balance' to the force, and this he does: during the prequels there are but three Sith Lords, but there are scores of Jedi. By the end of the prequels there are two Sith lords (Darths Sidious and Vader) and two Jedi (Obi Wan and Yoda). That's balance.
The origin of the Star Wars films is commonly held to be Akira Kurosawa's movie, The Three Villains of the Hidden Fortress--a title that, I think, is quite telling. The villains are the crucial part of Kurosawa's film, just as they are in the Star Wars saga.
Hidden Fortress belongs to a genre of films known in Japan as Jidaigeki, (note the similarity between 'jidai' and 'Jedi'): period dramas set in the Edo period. While the structures of the films are different, many familiar characters can be found in Hidden Fortress: the spunky princess; a general of great repute, now fallen on hard times; bickering peasants (droids)... and of course a trio of interesting badguys. Most interesting among them is Tadokoro, a former comrade of the general's who has changed sides. Tadokoro has been horribly disfigured as a result of a duel with the general, but in the end he seeks redemption by betraying his new master.
It's a great story, and no surprise that Star Wars grew around it, rather than the 'free-the-princess-stop-the-baddies' premise that it began with.
Star Wars is the story of a villain; make no mistake.
