It’s really hard to pick one.
There are so many cards I passed up in the 2010s because they were outside my goals. But the thing that kills me is having balked at how expensive certain cards were in that era.
“Imagine paying $1,000 for a Pokémon card. Ridiculous. You’d have to be diseased in the soul.” Now those cards are $13,000 and I can safely say I will never spend that much on a Pokémon card. Back then, the difference with the old prices was I wouldn’t spend that much money on the hobby. Now with the new prices it’s just not possible and will never be.
But if I could go back? I’d buy all those cards I balked at and then some.