Personally, I think the one Elvis CD I would recommend above all others--which would be extremely difficult to decide upon--would likely be different than the one CD I would pack for a cross-country drive.
In fact, I couldn't answer the question if posed that way.
But for the car ride, I'd probably take either the Promised Land or Moody Blue upgrade. I get quite a bit of mileage out of those two.
So many close runner-ups though!