I'm thinking the double-telescoping lightsaber Darth Vader figure is the rarest of them all. He was never actually released in mass quantities and only exists in salesman samples. Meaning that there are probably less than one-hundred of them in existence, not counting the salesmen who probably gave them to their kids who subsequently trashed them. Dumb kids, what do they know?
That should beat the pants off of Vlix, vinyl caped Jawa and YakFace as far as rareness goes.