Below, you can find and download (for free!) a country-level panel data file in long format (1985-2018), ready to be merged with all kinds of country-level data over time. The file lists 250 countries, dependent territories, and (other) special areas of geographical interest, as determined by the International Organization for Standardization (ISO). Years prior to 1985 can easily be added to the file, if desired. The file includes four ways to identify countries as well as six binary variables indicating whether or not a country is a member of several well-known international organizations (including the OECD and World Bank).
Hopefully, I have saved you some time. Let me know. Any comments or suggestions are very much appreciated!