Game Boy Micro, though, tbh, it was already like 5 years discontinued, I mostly regret it because it was still fairly cheap and always wanted one, maybe one day if I feel like saving money for it.
For the most part I don't think I regret something, I do miss when collecting Nintendo GameCube was cheaper because when I used to do it, the console was barely 10 years old, so while it wasn't new, it was still fairly common and some rare games weren't that sought.
I actually lived Nintendo 3DS I sometimes feel it died too soon but I can understand why it did, I want to mention PS Vita but I can't stop thinking that seeing dying slowly and even Sony forgetting it until stop mention it at all could have been a bit rough considering the machine wasn't exactly cheap at the time in my country, but reading old threads and seeing how the community kept the hype alive honestly seem like it could have worth it, is one of these things I do kinda regret missing out, just like 3DS, I kinda wish PS Vita lived a few more years, support I assume we all wishes it had more, ha.