Which Kriegsmarine defection would help the Allies most?

Which Kriegsmarine vessel or group would have been most useful if it defected to the Allies, and would the ship remain operational?

A submarine would yield valuable technical intelligence, while the largest operational gain would come from the destroyer force in Norway defecting with its crews.

Capturing a German submarine would give the Allies useful information about underwater design and equipment. The more consequential counterfactual is the German destroyer force abandoning the Norwegian campaign and reaching Britain with its crews and embarked troops. Once the troops were removed, the ships could be put back into service. They would probably operate in the Mediterranean or Indian Ocean, far enough from German-held territory to reduce the chance of a second defection. The scenario separates the intelligence value of one advanced platform from the force-structure value of acquiring an entire operational group.