In Generation II, Unown's form is determined by its IVs, with a quirk in the calculation making Unown-Z less likely to generate than other forms. This means Unown-Z is the rarest Unown form in Generation II, but this isn't the case in later generations, as Unown's form is no longer calculated with its IVs.
Individual values (Bulbapedia)
A unown forms are equal in rarity. The forms ! and ? of unown are considered the same in rarity as the others even though you have to do something in order to find them rather than just hope to encounter them in the wild.
