You can start with the latest release:
https://liferay.dev/blogs/-/blogs/liferay-portal-7-4-ga71-and-liferay-commerce-4-0-ga71-release.
Every release goes through the same rigorous testing process and
should be considered stable.
Also have a look at Daves blog post on how to handle weekly releases:
https://liferay.dev/blogs/-/blogs/how-to-handle-the-weekly-releases
which offers some good insight on how to handle upgrades.
