The board ate my last reply, so I’ll try to be brief.
I agree that Fuso and Chiyoda are probably the candidates for WW2, if we only look at warships. I would not be surprised if one of the merchant ships carrying prisoners back to Japan ended up being sunk with all hands and a much higher death toll. Awa Maru and Junyo Maru come close, I suppose. So does the SS Thielbek, in a similar role.
I wonder if the answer is one of the giant rowed ships that participated at Actium, Lepanto, or Salamis? Certainly, if one of the ginormous “Forty’s” of Philopator had sunk with all hands, it’d qualify, with it’s complement of over 7,000 rowers, sailors, and soldiers. I don’t believe they ever saw battle though, and I wonder if any were actually built, or if they were the antiquities version of military vaporware. While Antony’s navy at Actium certainly had ships large enough to potentially qualify if fully manned, malaria outbreaks meant that he had to sail with drastically reduced complements.
I had thought that some of the Spanish Armada ships would be large enough to qualify, but the wiki of their travails in Ireland suggest that even the largest ships (La Trinidad Valencera, 1,000 tons, 360 men) weren’t big enough. Maybe elements of the 2nd Mongol invasion fleet, the one scattered by the Kamikaze typhoon of 1281? An MA thesis on the fleet, and archaeological examination of its remains can be found here. He cites the Arab explorer, Ibn Battuta, who mentioned in 1347 that large ships, “carried 1,000 men, 600 sailors and 400 marines, had four decks and twelve sails, and were followed by three small vessels.” So perhaps one of those large ships, fully loaded, and lost at sea during the storm, might have been greater than Sydney, but certainly not Fuso.
Also wanted to mention the fates of HMS Neptune and USS Juneau. Neptune hit a mine in 1941 and lost 737 of her 767 men. 1 man of the 30 survivors that made it to lifeboats managed to survive 5 more days at sea until being rescued by the Italian Navy.
Juneau, while limping away from the November 13, 1942 battle of Guadalcanal, took two submarine torpedoes, exploded, broke in half, and sank in 20 seconds. Other ships in the vicinity, seeing the explosion, assuming no survivors of the sinking, and being engaged with the IJN at the time, left the area. Actually more than 100 men of the 699 man complement, including at least 2 of the 5 famous Sullivan Brothers, had survived the immediate sinking. 8 days on the open sea left only 16 survivors.