owncloud
Support 5 star rating
I'm using the Ultrasonic app on Android with the "5 star rating" option and it seems to work fine with the Music Subsonic API, but the rating is currently only visible in the app and not in the Music web interface. It would be nice to also support the 5 star rating option in the web interface in addition to the current singe star rating. The database already has the separate columns for "starred" and "rating".
The web UI currently has minimal support for the "5 star rating" in the sense that the Advanced search feature can find entities by the rating. But as the rating can be set only via the Subsonic or Ampache API, the feature is very much lacking, and probably seems weird to those users who don't use any external clients. So yes, proper support for 5 star rating is something I would like to add at some point.
Ample.fm may be used as companion music player. It supports 5 start rating https://github.com/rstefko/ample.fm.
@graphius Marking songs, albums, artists, and podcasts as "favourite" is already supported. You can find the "star" toggle button for this in the upper left corner of the details pane. The favorites can then be listed using the smart playlist with related filters or the advanced search functionality. A dedicated favorites view has been requested in https://github.com/owncloud/music/issues/779.