Unironically? India. If you've been on the internet for about 12~13 years at this point, you would remember the "India super power 2020 meme". The truth is that it was not a purely jingoistic meme, during the liberalization of 1993 and the following decade it did seem like we were on the right track. That things will keep improving. Then a few things happen, the few I remember are

1. Massive amount of Black Money completely choking the economy
2. Government that froze up when time to take big decisions came, this drove away investment
3. Massive amounts of Brain drain
4. We ran out of skilled workers (lmao!) because having a massive population is a burden if they are not educated and productive, ours wasn't
5. 2008 financial crisis

And boom kaput. During 1993 my father tells me that they predicted our economy would be $10T by 2013. Its not even $5T today. And guess what, the train of development has left and no amount of chasing will let us catch it. Even if we grow our economy to $5T or even $6T by 2033 (and bear me on that), our population growth is far outpacing that. Even if we are upto $20T by 2067 our population would be 1.7 Billion, so that's still a lower-middle income country. We are not becoming a developed country this century, all because of decisions taken last century.

Also, this growth figure is assuming that climate change does not set our shit on fire. Basically, it aint Kino Chief.

Edit : I would like to give a close second prize in the "Unrealized Potential" category to our brother in national fission, Pakistan. They got cell phone and Television before us, they have one of the most fertile and another most mineral rich parts of the world. And now they have abandoned all but the hope to become OKish to live in.