Starting with the one I miss the least, we've got the 1973 Topps Mike Schmidt RC I used to own:
This card was a huge piece of my collecting history - it was my first "steal" (at 75 cents when I was a kid) and, with two Ken Griffey Jr. cards, was the longest tenured card in my collection for a long time.
I don't miss it because of the collecting history it continued to make for me. I traded it for my first set of customs, which came out awesome. And then one of those customs became my best TTM ever...which is doubly awesome. So as great as this Schmidt card was in my collection for 15-ish years...it has done even better stuff for my collection since. If only all cards we moved brought on as much awesomeness.
One card I miss some, but not that much, is my Byron Leftwich Jaguars patch:
I sold it a few years ago at a flea market for like $10. It was a profit, and at the time the money was nice, but I sometimes wish I had this back.
Two other cards I sold that day, however, I miss more. These two:
I love Ray Allen, and that was a real nice patch card /75. I made a profit on it (again think I sold for $10), but I've never seen a patch that nice close to the same price range since then. So I miss that card (I think I was okay with selling it since he was on the Heat at the time).
As for the Expos card...man...selling that card hurt then and it hurts now too. I rationalized it in that I was getting $20 and that I didn't really collect any of the 3 guys on the card, but a Pedro patch that nice combined with a Randy Johnson that nice combined with a Javy Vazquez number patch, all on the same card, AND it's an Expos card (yay dead franchises) - that wasn't worth giving up. This is probably the #1 card I wish I still had, and proof that you always need to be sure it's worth the money when you're selling to make some quick funds.
If the Expos card and Ray Allen card have any competition for most missed card, I'd say the MJD patch card #'d 32/50 would be it:
Back when I dabbled in Jaguars patches, this was a huge get. It's got a cool college piece, it's got an amazing NFL piece (and an okay one), it's #'d to his jersey number, and it's MJD, a pretty cool guy. I only gave it up because the biggest MJD collector on Blowout was willing to trade me a nice Seneca Wallace plus a bunch of other hits for it. And while I did get some cool stuff in the deal, none of the cards were as cool as this piece, and if I had a time machine I'd go back, turn down the deal, and just wait out the Seneca card on Ebay.