I propose the Fairey Swordfish, not so much for its capabilities as an aeroplane but after Taranto and the crippling of the Bismark all of the worlds navies realised that the game had changed. Although an obsolete design it did have the ability to loiter about and carry a RADAR system which is the basis of anti submarine warfare.
I do like the old Swordfish, but I can't really see IT as the game changer. The vulnerability of ships to air attack was well known, in fact I would argue that as the war progressed navies realised that surface shipping was not as vulnerable as first feared, given certain measures, certainly the British thought so after the Battle of Crete, and Cunningham said so.
The game changers in anti submarine warfare were technological advances, not the aircraft that carried the electronic boxes. There were far better anti submarine aircraft than the venerable Swordfish.
The Swordfish did have one feature which as far as I know no other aircraft had. This was the attachment for the observer, the fantastically named 'anti cavorting chain'. For this alone the Swordfish should be at the top of at least one superlative list
Cheers
Steve