Worst? Mud Duck? Why?

Or are you actually talking about the Giant Mud Duck and its foam-board siblings? I thought the original 56" balsa Duck is a great little plane -- maybe not the best trainer, but way better than some of the flitty heavy little small trainers I've seen.

I don't have one specific worst in mind, but I've never seen a 1/2A "trainer" that was as good a trainer as any of the larger trainer planes. In fact, other than sailplane-like designs (tutor, twilighter, etc.) I've never seen ANY 1/2A that I'd use as a trainer, whether it's labeled "trainer" or not. It takes more than a high mounted wing to make a good trainer.