The rarest model of the Ford Thunderbird is generally considered to be the 1955 Ford Thunderbird "Porthole" hardtop convertible. This particular model featured a removable fiberglass hardtop with a distinctive porthole window on each side. It is estimated that only a small number of these "Porthole" hardtop convertibles were produced, making them highly sought after by collectors. The rarity of this specific model, along with its unique design elements, contributes to its desirability among Thunderbird enthusiasts.