OECD is a common list for developed countries. But did you ever really look at it? It includes included Mexico and Turkey with a ~10k per capita GDP. Even Latvia, Chile and Hungary @ 15k USD seem a bit underdeveloped. Czech Republic and Slovenia at 20k per capital are borderline but probably fine. Granted things change somewhat when you look at PPP, but large PPP differences generally mean an underdeveloped economy.
https://www.oecd.org/about/document/list-oecd-member-countri...