[Old thread, I know, but one that didn't get enough responses!]
My favorites:
Battleship - IJN Fuso; she feels like some old dragon crawling out of her cave to do battle one last time.
Battlecruiser - RN Hood and IJN Kongo; ships designed along the lines of HMS Tiger are, in my opinion, among the most elegant capital ships put to sea.
Heavy Cruiser - if you count pocket battleships, then those. I used to play with "contact" markers that only indicated relative ship size up until we reached visual range or got scouts out that way. The look on the Brit players' faces was always priceless when a cruiser blip became a pocket battleship. Think Obi Wan Kenobi's line "That's no moon..."
If you don't count pocket battleships, then RN Norfolk or Kriegsmarine Admiral Hipper. I appreciate the CAs that look like battleships if you don't have something nearby for scale.
Light Cruiser - USN Brooklyn for the stupid amount of guns she carries.
Destroyer - USN Porter [EDIT: I meant Clemson]; 'cause I like ships with four funnels...or sometimes the Japanese destroyers for their wicked torpedoes.
Subs - German Type VII, since I used to play one in Aces of the Deep.
Planes - USN Avengers or RN Fairey Swordfish...both because they look cool.