Since you asked....
Scale 1/350. Even though I can't really afford them, and don't have the room, (hopefully a temporary situation in both cases), I really like the large ships. Helps my fat fingers and fading eye sight build the kit better than the smaller 1/700.
Time frame World War II. While I am very interested in World War I naval subjects my heart for some reason belongs to WWII.
Subject, CRUISERS! I have thought for as long as I can remember that cruisers get the short straw. Most every other type of ship, Battlewagons, Flat Tops, Tin Cans, Pig Boats, have a nickname, cruisers don't. Cruisers, IMHO, were the real work horses of the fleet. They did it all. They stood in the line of battle and slugged it out with BBs, they escorted convoys, and engaged in anti shipping raids, shore bombardment, anti sub work, the works.
Nationality British is my first choice, but really any of the cruisers that don't get made too often. That means just about any cruiser except the Prinz Eugen.

The Italian Zaras are done, but there are so many great ships that have never made it to 1/350.
Holy Grail? Up till this year that was easy, a GOOD 1/350 HMS Hood. (A good 1/35 US halftrack was my other one but that's been done also. HOORAY!) My current H.G. is really more of a pipe dream. I know it's something I'll never be able to do. I would REALLY like to model all the ships in what I consider the most dramatic sea action of WWII, the 1st Battle of Guadalcanal, Nov 13-14 1942. For that, in 1/350, they'd have to come out with a whole SLEW of ships, both US and IJN, of all types so a lot of people would get their hopes answered.