Hello Music can download from the Youtube app via the share menu – and lets you list, display and open music from within the app.

Features:

Download video in MP3 from YouTube app
Material design
Browse Songs, Albums, Artists
Create and edit playlists
6 different now playing styles
Homescreen widgets
Browse device folders
Dark theme and UI customisability
Gestures for track switching
LastFM scrobble
Android Wear and Android Auto support
Playing queue in notification (Xposed)
Lyrics support
Chro