While it may seem unrealistic, perhaps farfetched before the advent of nuclear weapons, to categorize a single warship as strategic, one ship can rightfully be so described: the German battleship Tirpitz, the "Lonesome Queen" of the north during World War II, which by her mere existence influenced the balance of seapower in every ocean.
Langenberg, William H.
"The German Battleship Tirpitz: A Strategic Warship?,"
Naval War College Review: Vol. 34
, Article 8.
Available at: http://digital-commons.usnwc.edu/nwc-review/vol34/iss4/8