Some may argue that a psa 10 first ed charizard might be the rarest where as the Pikachu illustrator is the hardest to get so it makes it expensive/the rarest
But with your own opinion what would you consider the rarest card in existence that actually exist (pre release raichu doesn’t count)
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.
I don’t think anyone is making that argument.
The most popular card, but far from the most rare.
For a list of the most rare - check out this page: www.elitefourum.com/t/master-list-of-the-rarest-pokemon-cards/28390/1
We have a list of the rarest cards (rarity in terms of total amount released):
www.elitefourum.com/t/master-list-of-the-rarest-pokemon-cards/28390/1
Edit: PokeNastic beat me to it.
Welcome to the forum.
First and foremost, if anyone argues a PSA 10 1st ed zard is the rarest id stop talking with them.
Second here is that rarity isn’t subjective. Rarity comes with actual facts behind it and numbers that can be measured against that of mass produced items making them far less in numbers and difficult to obtain. So in retrospect here, in order to form an answer to this question youd have to have that knowledge of those super rare trophy-esq cards, consider extra copies, and then go from there to find an answer.
I dont have that information myself or else id just share it, but if youd like to dig through bulbapedia and check releases youd be off to a good start, and thats where you can gain a better idea of such things.
Rarity is dynamic! You can make any card, even the PSA 10 1st Charizard, the rarest card in terms of population report (and in case of Illustrator also in terms of copies in existence) just by buying the remaining 120 (38) copies and burning them.
As you can see from everyone’s answers, E4 does not mess around with sharing knowledge. Welcome to E4 Amigo. We’re glad to have you 
Calm down there, Kaiba.
I have a 1/1 fossil magmar with bbq sauce on it from when I was a kid. I’d say thats the rarest card
Rising Rivals Weedle is still pop 0 in a 10 in both reverse foil and non-holo IIRC, I hear the bounties on someone grading this thing are now in the eight figure range.
Roger, man of Ahhiya.
every card is the rarest card because there is no other card that is exactly the same in every way :’)
Just like you 
Rarest signed e4 member card. There are 2 Coop rookie cards floating around
Gold Star Greninja is fairly rare, buy up fast!
I saw it once but can’t find the picture, wasn’t there a child through the make a wish foundation (or something) who actually received his/her own Pokémon card? (meaning it’s registered as a legit card).
Technically that has to be the rarest card or tied with a card there’s only 1 copy of, anything that beats that would be rumors of a card that may or may not exist.
That was a Yugioh or DBZ card, not Pokemon.
it’s not the time yet. 
Ah ok, I see it now. It’s a Yugioh card with a dragon ball character on it (I think Goku), interesting!