In general, using the newest version possible is best.
Why Use a Newer Version
The Unity game engine tends to improve with each release (both in features and bug fixes). When Unity releases a new build, they’ll announce the features and bug fixes in it.
Unity has auto-converting utilities that will upgrade older projects to the newest version. If you’re concerned about upgrading being harmful to your current game, you can always commit your code, test the upgrade out, and revert if necessary.
Unity does not have auto-converting utilities to move newer projects to older versions of Unity. This can be a hassle if you’re trying to support older versions with your assets or if you’re working with other people that use older versions of Unity.
Generally, assets support everything newer than a certain version of Unity. Many assets require 2017 or newer and a few support 5.4.
There are relatively few users still using versions of Unity 5. Most users are keeping up with the latest version.
If you’re starting a new project, use the newest version of Unity. If you’re not, save your project and then upgrade!