Short answer in most cases you should download the latest version of CiviCRM.
Details: The main CiviCRM Downloads page offers a choice of downloads based on which content management software you are using with CiviCRM (Drupal, Joomla or WordPress). Generally, you will also have a choice between the 'current' version, and the 'long-term support (LTS)' version.
The current version is the most up-to-date. It is the version referenced by the current documentation, and contains the latest features and improvements.
The long-term support (LTS) is a slightly older version but still receiving maintenance and security fixes. It is created for the benefit of organizations looking for a stable release, and/or who do not have the resources to upgrade to the latest version.
The localization files are required for any site using a language other than English.
Older and pre-release versions are available for download on SourceForge. Look there if you want to help beta-test the upcoming version or need an archived version.
Release announcements on the blog provide a summary of key new features in each new release.