I found this to be a tougher choice than I thought. I love the design. The original is a classic in every way. Knowing that Ford added the conti kit in an effort to increase trunk room makes the '56 tolerable. Ford found a way to store the spare tire upright in the '57 and did away with the conti. The hardtop was standard on all three with a cloth top made optional. The porthole was a no-cost option in '56 and became standard in '57. You can see echoes of Ford's '58 designs in the front bumper and grill of the '57 as well as in those vestigial fins at the rear. But that line that runs across the top of the fins serves to visually elongate the rear quarters, which always felt a little odd to me.
The cars were not designed to be sports cars. They were more of a boulevard cruiser with an electric seat, push-button inside door handles, and a few other luxury touches. As such, they were not really great handlers. If you've ever driven one, they weren't all that easy to get into or out of. Once inside, they weren't all that comfortable and, with relatively flat seats, could be a bit fatiguing on a long drive. I had the privilege to drive a '55 from Las Vegas to Virginia. It was fabulous to be seen in, but not so great to be in.
All that being said, I vote for the '56. I like the period correct conti kit and the extremely tasteful, elegant design. Thanks for bearing with me as I thought this through.
Love all three! Back in 1956, the continental kit was in, and I was around to witness it firsthand. So, first place is the '56. Second place goes to the '55.