easy. Surcouf. Notice the frecnch flag.
It suddenly became famous in 2005 when the movie Loreli was released. The movie is about a German sub based on the Surcouf brought to japan.
http://wgordon.web.wesleyan.edu/kamikaze/films/japanese/lorelei/index.htm
Bravo, my dear Herr Major: indeed, our mysterious vessel was the world’s largest pre-WW2 submarine - Surcouf - armed with a pair of 203mm cannons, and an airplane. Her exact fate still remains a mystery, and her wreck has never been located.
Offically accepted explanation is that she went down on the night of February 18, 1942, having collided with an American freighter Thompson Lykes.
On the other hand, naval historian James Rusbridger in his highly intriguing book Who Sank Surcouf examined some more peculiar theories on her demise. All are easy to dismiss - except one. He states that the records of the Sixth Heavy Bomber Group, which operated out of Panama, are containing direct statement that they sank a large submarine in the early morning hours of the 19th of February, 1942. In view of the fact that no Axis submarine was lost in that area on that date, it could only have been Surcouf.
Perhaps an additional US exploratory naval expedition - similar to that one which had been led by oceanographer Dr. Robert Ballard (who discovered the Titanic back there in 1985) - will be able to find, investigate and photograph Surcuf’s remains…
And please, proceed – next turn is completely yours! 
guess this sunken beuty
hint: the ___est vessel of WWII
my favorite ship besides the U-XXI and Surcouf
and that was also a hint…
another hint:
it could carry planes
and no it is not the U-cruiser.
its a IJN’s three barrel-25mm AA
as you comparing with sourcouf and type XXI
i would vote for I-400
ah… close!
i-401,.
now i want my candy
you can have your candy!
And I-401 it is!
KMS Graf Zeppelin
I would like to ask the person that last answered correctly to advance this thread, because we have been sorely in lack of any good naval enigmas for the last few months.
If not, I ask the initiator of this thread to do the same, so we could get back to our game.
Wings, you do it.
All right then.
Ladies and gentlemen, please be kind enough to answer this easy question:
What is the name of this particular vessel?
Italian aircraft carrier Aquilla, while rusting in La Spezia in 1951, my dear Mr. Wingsofwrath. Obviously, carriers were very important vessels for you lately! 
Very well spotted my dear Mr Librarian. Really, nothing gets by you, does it?
And yes, I do have a certain fondness for carriers, especially those that have been unjustly forgotten.
Good, my dear Mr. Wingsofwrath! In that case, we will have many highly interesting moments in this thread.
However, numerous highly intriguing vessels of different classes are still waiting for their hard-earned presentation.
Take for example this one: without any doubt, it was amongst the finest ships in its own class. Furthermore – this ship actively served until the very end of the WW 2 in Europe!
What a beauty!
Quite easy, my dear Mr Librarian.
French light cruiser “Georges Leygues” , of the very sucessful “La Galissonnière” class of cruisers.
I can tell you this picture was taken after the 1943 re-fitting in Philadelphia (note the two US Bofors 40mm AA guns visible in the picture) and that is certainly “Georges Leygues”, because her sistership “Gloire” had the rear antenna mast mounted behind rather than in front of her funnel, had a diffrently shaped bridge and spent most of the war in “razzle” camouflage (although some pictures exist whit the ship in two-tone after the end of WW2) while her other sistership “Montcalm”, albeit having been re-fitted at the same US shipyard had a different shape antenna mast mounted on her funnel.
Ship class is right, but the vessel name is wrong, my dear Mr. Wingsofwrath. 
Oh. In this case it must be the “Montcalm”.
The only pictures I have of her show the vessel after the war and indeed the rear antenna mast is different, but maybe she initially looked exactly like “Georges Leygues” after her refit in Philadelphia.
In any case she can not possibly be “Gloire”, and the other ships in her class (“Marseillaise”, “Jean de Vienne” and “La Galissonnière”) were scuttled in 1942.
Yes - it is “Montcalm”, my dear mr. Wingsofwrath. Besides – only she carried out shore bombardment on targets in French Riviera on April 23rd, 1945.
You have earned your well deserved turn. Therefore please, proceed.