Impossible to answer.
Do you mean rarest as in fewest number of copies ever produced? Do you mean rarest in terms of PSA population or hardest to find in mint condition? Do you mean rarest in terms of hardest card to acquire? Do you mean rarest as in card that requires the largest amount of money to purchase?
This is all to say that cards can be “rare” in different ways and there isn’t one correct way of determining whether a card is rarer than another. You could make a very solid argument for about 50 different conceptions of “rarity.” As an example, many people would say the Illustrator is the rarest card even though there are cards which have fewer copies, cards which are harder to find, and cards within the same rarity tier that have lower graded populations.