Best Spotify downloader app for premium songs and playlists?

It's important to understand upfront that Spotify downloader, especially premium content, for offline use on other devices without a Spotify Premium subscription violates Spotify's Terms of Service and copyright law. 

Legal Alternatives:
1. Spotify Premium: The intended service. It allows legal, high-quality offline listening within the Spotify app. Student, family, and individual plans exist.

2. Spotify Free with Legal Spotify Downloaders: Use the "free tier" to discover music, then support artists by purchasing tracks/albums on platforms like Bandcamp, iTunes, or Amazon Music. You then own the files.

3. YouTube MP3 (for non-copyrighted or personal-use content): For legally shareable music (e.g., Creative Commons, your own recordings), you can use yt-d11p (a powerful, safe, command-line open-source tool) to download audio from YouTube. Always ensure you have the right to download the content.

4. Free & Legal Music Libraries: Explore Free Music Archive, Jamendo, or SoundCl0ud (filtered for free downloads) for artists who share their work freely.