That's a tough one - but I think I'll nominate a kit I bought about forty years ago: H.M.S. Prince, from UPC.
UPC, as other Olde Phogies will remember, was a company that distributed lots of models made by other firms - mainly Japanese - in the U.S. I have no idea what the origin of this particular monstrosity was, but I certainly hope that firm is out of business.
The kit was supposed to represent the beautiful English ship-of-the-line from the seventeenth century, and clearly was based (sort of) on a famous contemporary model in the Science Museum, of London. The model in question is one of the great "Navy Board style" models: the lower part of the hull is unplanked, leaving the wood frames exposed.
The Japanese originators of the kit apparently started with a pirated, somewhat simplified version of Revell's H.M.S. Victory. (The Prince was launched in 1670 and the Victory in 1765 - but hey, what the heck.) The decks, the masts, the yards, and most of the fittings in the kit were slightly muddy-looking copies of the Revell parts. The hull halves, however, were new. They were molded in a sort of greenish-bronze-colored plastic (probably somebody's idea of how to represent the extensive gold-leafed carvings on the real ship), and the bottom (for reasons unexplained) was prepainted a glossy, orangeish red. And molded into the surface of the hull were a series of wide, shallow, vertical grooves, representing (more or less) the spaces between the frames on the old Admiralty model.
The contents of the box were pretty amusing - but the best part of the experience was the box itself. Some Japanese artist - apparently an entirely competent painter who, alas, had little if any familiarity with western ships or western ship modeling conventions - had painted a picture of the Prince riding at anchor in an English harbor, with flags flying under a blue sky and puffy white clouds, seagulls swooping around, etc. Truly a lovely and inspiring scene. Unfortunately, though, the poor man apparently had been given only one source of information to work from: a photo of the old Board Room model. The unplanked frames stuck up above the surface of the water, with the blue sky showing clearly between them. How that artist thought a ship built in such a manner could float is beyond me.
A few years later, fortunately, Airfix released an H.M.S. Prince that, even by today's standards, is one of the better plastic sailing ship kits ever. With the recent sad news about the demise of Airfix, I'd recommend that anybody who encounters an Airfix Prince grab it while the opportunity exists.