Couldn't pick exactly one, but the several dozen bottles of various potions, powders, lotions and whatnot you're supposed to lather on babies. We only use that anti-thrush bum cream.
The curse of the muslin wraps. Some were big enough to use as headscarves, most we threw out.
Breast pads. Was given several boxes, used two. Two pads, not two boxes.

Our baby bath actually fell apart from TOO MUCH use - baths for two babies (far too cold in the bathroom for babies), then used as a washing basket, and a boat, and a pond, and I think we mixed cement in it once. Well worth the $10 or so from Ikea!