GeoServer 2.9.0 is the new stable release suitable for use in production. This release requires Java 8.
Important updates:

• Web admin application layer, preview, global settings, image processing and raster access screens have been updated.
• User guide has been restructured for ease of navigation and to reduce duplication.
• The REST API can now be used to manage icons, fonts and configuration files and report on the status of installed components.
• Internally this release features Wicket 7 and Spring 4 and the latest JAI-Ext library.
• Layer bounds can now be generated from spatial reference systems extent
• Styles can now be configured with a custom legend

For more details please see the blog post http://blog.geoserver.org/2016/05/30/geoserver-2-9-0-released/ and release notes (https://osgeo-org.atlassian.net/secure/ReleaseNote.jspa?projectId=10000&version=13003.