Kind of an odd question, but does anyone know what the rarest or hardest card to find that is in a set excluding secret rares and first editions? What about promo cards that are part of a set (prerelease gyarados , e3 pikachu, etc.)?
Korean unlimited base cards that can not be obtained in theme decks would be my guess
Prerelease Raichu. 
I don’t really understand your question OP… You want to know THE rarest set card? There are plenty of different variations. I would put Trainer A/B deck cards up there.
Prerelease cards aren’t super rare. There’s thousands of them printed.
The TV Reporter (EX Dragon) reverse holo was printed at a significantly reduced volume from the other uncommons. Also Oracle from skyridge because it was once desired by deck builders.
Promos #40 & 41 due to limited distribution, and same for the red cheeks E3 Pika. The 1st ed. #1 & 17 promo were both a short print… the Persian errors were shortly recalled. Those are just a few off the top of my head 
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