Just because you're on-premises today, doesn't mean you will be there tomorrow. And even if your local laws and regulations make it impossible for you to move to the cloud, there are benefits in extending SharePoint in a cloud-ready way.
What version of the SharePoint Framework should you use for your project? 'The latest' seems an easy answer, but there is more to it.
While SharePoint Framework has been around for a while now, it could be that you're just starting with it. Here is the easiest way to get going.