If you mean distribution, and the most literal sense of the word “rare”, then probably the Female Battle Road Trophy Card, which was awarded to the only female winner. Here is a thread from a few years ago with the tournament information.
The 1997 Pikachu Trophy cards are extremely rare as well. There were 4 copies of each awarded.
Typically the word “rare” is not always used in the literal sense. It is commonly used in relation to the overall demand of a card. This is true in all hobbies. Here is a list of baseball cards rarer than the T206 Honus Wagner, but are nowhere near the value. Even when you search “rarest baseball cards” the term that you see more frequently will be “most valuable”, as that encompasses rarity, history; overall demand.
A great example of this in pokemon are those distribution promo cards. They have an estimated 6-12 copies, but are common illustrations, the only rarity factor is the stamp. They are almost as rare as the 1997 pikachu trophy cards, but realize a fraction of the price.
Oh and The PreRelease Raichu has a minimum of 12 copies. This is information that will most likely never be properly confirmed with documentation. As time goes on the amount of reliable sources are less involved and/or inclined to share.