For me, it’s the ‘cards’ that were included in the sealed WotC Black Star Promos (most of which are featured here I believe).
I have had the Ancient Mew details cards for years (have it loose and sealed), but that one was pretty famous. I got the Neo card (with Unown and Pichu on that came with Entei) loose in bulk about 10 or so years ago, but never identified it came with a sealed promo until a year ago or so.
But the rest, including the ‘Catch Mew!’, ‘Master the game!’ and ‘Legendary Birds’ (and their slight variations), I didn’t even know existed until recently! I’ve since added them to my database.
PS: My first post here on E4! I’ve been a lurker for years, and been involved with the TCG community on reddit for many years too, but I figured I might start focusing my attention on here as well, as other users I’ve spoken with in some capacity are also in this forum.