Not so much a regret, but for me it is the Air Arms TX200. I'm not faulting the gun. Obviously, the gun's accuracy is not in question with a competition pedigree like the TX200 has... for my purposes though, and at the distances I'm shooting at, I just could not get any consistency with the gun. My gun's main purpose was a backyard pesting rifle shooting out to around 35 meters or less. I don't know if it's the cold barrel accuracy (or lack thereof) but my kill percentage with that gun when I owned it was only around 50%. I've tried every sorts of hold, tripod, no tripod, bean bag, no bean bag, holding as firm as I can, holding as light as I can, but I just could not be consistent with that rifle at all. Now, the TX200 was my first air rifle (although not my first venture with guns in general) and I probably could have been more accurate with that gun if I put in more time and work in learning it, but with how demanding the gun was to get consistent with it, I was just not willing to bend over backwards for it to have it shoot the way I need it to. It was just not great for how I'm shooting the rifle, i.e. shooting at small pests from out the window, sometimes at awkward angles, mostly at 25 to 30 meters. If most of my shots are within 15 meters or less, I probably would have a much better experience with that gun. Again, I'm not blaming the gun as hunters have done amazing things with the gun... For me though, the effort it required is just a bit too much. Maybe if it was an heirloom I would have been more patient with it.
Now, 3 years later since I sold my TX200, I'm thinking of buying another springer, HW95 to be specific, but my negative experience with the springer makes it hard for me to get another one again.
Edit: I just looked at my logs for when I owned the TX200. I had 34 kills with the gun in the 1 year I owned it. My confirmed kills percentage with the gun was actually 72%. Meaning I killed 72% of the animals I shot at. However, this doesn't take into account the multiple initial shots I took at some animals due to complete misses or wounding shots which are then followed by kill the shots.
Take the misses and the wounding shots into account and my kills-to-shots ratio is only about 1:2. This means that only 50% of all the shots I took at animals with the TX200 are confirmed kills.
The other side of that 72% kills are either a complete miss and the animal escaped or unconfirmed/unrecovered/wounding shots.