Stranger Things. I think more so because I have a very small collection and was a day 1 adopter. Having only 2 total pins at the time I quickly realized I dont want to wait on code, etc. I would play the hell out of it if I had one now though.
For me regret is a strong word, because I have learned to look at pinball as a hobby where you can trade if you dont like something. Pinball doesnt seem to tank in value 24 hours after purchase like many other things/hobbies. I ended up trading Stranger Things for a NIB JP Pro, and a couple hundred bucks. For me it was worth it having the game for a few months in my possession.
You want real regret, buy a boat.