In your earlier thread of ‘What are your top 10 favorite cards?’, I already had trouble posting just one comment: one & two.. 
But if I could only keep 10, though. 
Honestly, I have no freaking idea.. My collections are collections, because they’re hundreds of cards combined. If I could only keep 10 cards, it no longer is a collection to begin with.
Of course I prefer to never get rid of the majority of my cards. But if someone theoretically would offer me 1 million USD each for certain individual cards, there honestly wouldn’t be any card I wouldn’t get rid of, even childhood cards. 
Anyway, to at least give some sort of answer, these few come to mind:
This is my favorite TCG artwork, and this Moltres copy in particular I received at cinemas as a kid.
Seviper is my favorite Pokémon, and this was the last card I added to 100% complete my Seviper collection in all languages (excluding new recent releases after I completed it). This unlimited edition Japanese WCP Seviper is also the rarest Seviper card among all of them, with an interesting backstory of why it is so hard to find.
As a Pikachu collector, I love that they’ve made this card for the 25th anniversary. It’s so cool and unique to see 25 different illustrators collaborate on a single artwork, and I also love that each of those illustrators commented on it on the Pokémon website at the release of this card.
I would want to keep at least one copy of the many different Base Set Pikachu variants. And since the Ghost/Phantom stamp error is so unique, including it being a Shadowless card, it makes it the perfect candidate for that purpose.
These four cards are the 1st; 500th; 1000th; and 1500th Pikachu cards I added to my Pikachu collection. Of which I especially want to keep the 1st and 1000th ones. 
(Btw, I’m closing in on the 2k mark, although it’s defintely going a bit slower since I stopped collecting all languages and primarily focused on English releases - I’m at 1949 at time of writing, fwiw.)
And that’s about it I think. There are dozens of other cards that come to mind btw. Some from my youth; some from my little brother’s collection that I took over; some with interesting stories; some very rare cards; some gifts; etc. etc. But there are a lot more than just ten of those (or four I should say, after subtracting the six already mentioned above)..
Greetz,
Quuador