I voted for ESB, just because that was his most badass incarnation. Hoth invasion, driving star destroyers through asteroid fields, executing inept officers on the spot, telling the galaxy's most lethal bounty hunters what to do, owning Lando, torturing Han Solo, using him for a guinea pig in the carbon chamber, then beating the living crap out of Luke, just to mind^^^^ him sideways by telling him he's his father.
I know a lot of you guys have the Tantive IV appearance seared into your brains, but I was only 4 when I saw that in the theater. I saw ESB when I was 5, and it left a much deeper impression.
Plus, an ESB Vader could come with a meditation chamber.
I don't really care which one comes first, just as long as they do one for each film. I'd be irate if I had to settle for the same Vader duelling Obi-wan, ESB Luke and ROTJ Luke. Irate, I tell you. Like the Satan faces in mushroom clouds.
If they do a ROTS version first, I hope it comes with droids, an operating table, and a full reveal 3-part helmet.