I think that would be 90 or the Joker covers
Are we talking rare or desirable here? Sometimes two sides of the same coin, sometimes not. I had difficulty finding a #98 until a boardie was kind enough to trade me one. Now I realize it isn’t as rare as I thought! I’ve always thought these may have seen variation in regional distribution, that pic reinforces that for me.
Desirable in the sense that more folks want them would be the list quoted above. #90 and the Joker covers. None of those seem particularly rare, but more folks are interested.
Personally, I’m missing #92, 95 & 106. Not key issues by any stretch, but they do seem to be the rarest WW DCUs in my experience. YMMV