Every gun I've sold I wanted back at some point or another, and at the same time, on some level I don't really regret selling any of them (they made sense at the time).
The ones I most regret (or regret most often):
-Mossberg 500 Persuader. I loved that shotgun, but I really needed the money. I replaced it a few years later with a Benelli Nova 18.5" with ghost ring sights. I couldn't bring myself to buy a gun I had sold at a loss, why pay more than I paid for my old one new when I sold it for even less (that just didn't seem to make sense), so I went with something different. I hate my Benelli, though there isn't really anything wrong with it. I'll have to get another Persuader or a 590 one of these days.
-My Taurus 82. It was my first handgun so I should have kept it. However, at the time I only envisioned myself owning 4-5 guns (auto, revolver, a couple rifles, maybe a shotgun) and I had just bought a S&W 65LS. The 65 was a .357mag and a medium frame revolver, so it was the size of the 82 and could shoot the 82's .38spl round. I wanted the 65 because I really wanted a .357mag. So, I traded the 82 and another gun (a Ruger KP89) on a Charles Daly 1911. Now that my gun collection has been over a dozen for years (and was within a couple years of the sale) I really do kick myself over this trade. The only reason I'm OK with this sale is that my Daly started my love affair with 1911s, but I really wish I had my first handgun (and I would like a K-frame sized .38spl in general).
-Ruger 10/22. I didn't like it as much as I should have/wanted to. It was a decent gun, reliable, pretty accurate, but whenever I compared it to my Marlin 60 I really liked the Marlin (though my Marlin could have been more reliable) and the Ruger really felt like an appliance. So, now I'm one of the few gun enthusiasts without a 10/22 (I often think about fixing that situation since the 10/22 is so affordable).
-Ruger KP345. I don't really regret the sale, I regret the KP345 didn't work out for me. I really like the design of the gun and the idea of the gun. I think it was a huge step in the right direction for Ruger. However, the initial execution of the design was flawed (a lot of the early buyers had major issues with their guns, from what I read I think that has long ago been alleviated). It took a while but I finally got my gun to be reliable "enough," at which point I sold it because I would forever be worried about when the next shoe would drop and it would become an unreliable POS again.
-CZ 40B. I was only lukewarm to .40S&W, I originally intended it as a CCW for when I was somewhere I can carry and it was a little on the large side, and for several other reasons, none involving anything actually wrong with the gun, I ended up selling it. I like CZs a lot, now I like .40S&W almost as much as I like .357mag or .45ACP (and a bit more than I like 9mm and .38spl). The CZ 40B isn't made anymore. I have shopped for one from time to time but can't get myself to pay more than I did for a new one (sometimes significantly more) for a used one. I really think selling it was a mistake. The only thing that keeps it from being an active regret is that it was so long ago I rarely think about it.