For me, it was my PSA 9 Espeon Play Promo. Its the 07 variant and the crown jewel of my collection.
With my collecting journey, the hobby predominantly funds itself, so i sell/trade to get the cards I really want. And it took around $27k in trade to grab.
I take great pride in the work I’ve put into getting cards like this into my collection. The linegage of acquisitions behind what got me this beauty. And I’m excited to see where it may take my collection in the future.
The Japanese gold stars for the eevelutuions always look so much better
A 9.5 CGC 1st edition Raichu (that magically became a 10 overnight) at around $1600.
Most other slabs I’d be interested in don’t come close to that at all so I don’t think it’ll be beaten in my collection unless I start a go fund me for a Jasmine’s Raichu.
My biggest transaction was a sale!! Which is always a good thing 
I sold my PSA 9 Alola Friends for 6600 CAD back in June. I hit close to the peak on the card so I’m happy with that!
My current savings techniques don’t allow me to spend close to this price on an actual purchase but Maybe one day!
My top three most expensive cards bought thus far:
My top three most expensive cards sold thus far:
Greetz,
Quuador
Beautiful grail. Congrats on acquiring this! I’ve told myself if I was ever going to put some major money into the hobby, that this would be one of the cards I go for. The Japanese variant is just so cool looking with the holo and white border. Maybe one day lol.
As for me, my biggest purchase was a BGS 10 Present Campaign Rayquaza for 1.6k USD. This remains the only card I’ve spent more than 1k on, but I have some particular cards I will gladly pull the trigger on if the opportunity arises.
The most Expensive card I have ever bought was Shining Raichu PSA 9 for $130 in 2019, it is still the only time I have spent over $100+ dollars on a single card in my 6 years back in the hobby i represent the low budget collectors. Also bought it in Ebay from TCA Gaming
My 3 biggest purchases were:
My biggest transaction was selling my PSA 10 Charizard Gold Star in 2018 for 4.5k. But I don’t like to think about that.
Great card! What do you mean by the 07 variant?
This is my biggest purchase for a single card. Previously, it was my PSA 10 Crystal Lugia that I bought for around $900.
Amazing price on the lugia
The most I’ve paid for a single card so far is ~$90 for a CGC 8 Natta Wake Hoppip. I’m not super comfortable paying close to $100 for singles I’d consider myself on the more budgety side of collecting.
I think this question is kinda getting at personal preference, as apposed to what market price you’ve been willing to jump for, so…
In my collection, I paid $350 (?) for this a few years ago. There are few cards I’d hundreds or thousands for, for my collection.
i think the most i’ve spent on a singular card was ~550 for boxy karen to bring to hartford earlier this year! though i’ve had larger multi-card transactions for both selling & purchasing 
So this card was distributed from 05-07. 05/06 variants have a different color or saturation level, as well as the border size on the back being thinner than the 07s (the fade from brownish to blue extends farther into the surface of the back on 07)
The 10 on the left is an 07 where the 9 is a 05/06
Pics courtesy of crystalcollector off IG
I’ve never been one to spend a lot on individual cards. In the world of misprints you can get very expensive cards for very cheap if you keep an eye out 
10k
Wow is the German Birthday chu just really rare? That’s a steep price
