Hard to narrow it down to just 1 card.
My 1980 Henderson rookie was perhaps the first big money card I ever got. I spent a whopping $8 for it, which is still the most I've ever spent on a single card. I've since set my rule at $5 as a max for any card.
My 71 Roberto Clemente card was perhaps the hardest one for me to find under my $5 limit. I was thrilled to find a version in good shape for just $3. Because of how long and hard I searched for that specific card, it will always have a special place in my collection.
I have a 63 Yogi card that my son gave me as a birthday gift that is also special to me. He decided that he would start giving me a card every year for my birthday that was one year older than the prior. He started with the 63 thinking it would be the oldest in my collection (it wasn't but he didn't know that at the time).
The Jeter card with Bush and Mantel on it is up there for me as well as I always found that one funny.
My 92 DJ Derrick Turkey Bowler card (which I received from a member here in a trade) is up there as well, considering how obscure and bizzar that one is.