Gosh, I'm not sure I have any really. Perhaps plants I personally just don't like eating. Cherry tomatoes do amazing in my climate, but I hate eating them, so perhaps those.
I think most of my "regret" plants would be me making mistakes, mostly by trying to grow them out of season.
Mine are in no particular order: warrigal greens (didn't enjoy the taste, extremely vigorous), vietnamese mint (nearly blew my head off with the heat, extremely vigorous), flat nectarine varieity (why did I think a half sized fruit would be good???).
Anything that grows here is a miracle in my dry climate so i regret nothing. If i don't eat it something else will, its all more life which i appreciate. In terms of least favourite i'd probably say Tromboncini and button squash as they don't taste anywhere near as nice as butternut pumpkins. I don't really like the little red radishes either. Pigs and chickens can be a nuisance when they break into the garden, or if you have mean individuals but they still taste nice.
I don't have any plant regrets, but I do have garden bed regrets. I put a lot of effort and some money into building a garden bed in my front yard, planted some perennials and some annuals, figured they're just flowers, I don't need to be harvesting them so I can just let it go and grow. Now 12 months later the annuals have all died back and their remnants need to be removed and composted, new annual seeds need to be planted in a couple of months, the bed has become a bit tattered looking, and I can't bend over well enough to get it cleaned up and ready for winter least of all spring. So come July I'll be pulling it all apart and putting in new raised beds so that I won't have to bend down as far to deal with them and can better look after whatever I end up growing in them. The only plants that have survived are ones I want to keep, and on the bright side, it gives me a chance to redesign the yard, rearrange the remaining plants, and put in a lot of new ones.