The KMS Bismarck, of course.
The idea here is that, for whatever reason/reasons, the captain assigned to the Bismarck is more disaffected with the regime than OTL. (He seems to have been at least associated with people critical of the regime.)
As such, he defects with his entire ship (and possibly the Prinz Eugen as well?) some time after the DDs have to return to Germany.
Is that doable? And what would the knock-on effects be (apart of course from a monumental propaganda coup and Hitler eating a carpet!)