You can download the best video and audio by using:

youtube-dl -f bestvideo+bestaudio "link to youtube video"

If that gives you an error, try the following instead:

youtube-dl -f 'bestvideo[ext=mp4]+bestaudio[ext=m4a]/bestvideo+bestaudio' --merge-output-format mp4 "link to youtube video"

Best MP3 audio

youtube-dl -f bestaudio --extract-audio --audio-format mp3 --audio-quality 5 --embed-thumbnail --add-metadata link-to-your-video

The –audio-quality 0 uses the highest encoding quality but can increase your file size unnecessarily. The default is 5 and might be a better choice depending on the source quality.

youtube-dl -f bestaudio --extract-audio --audio-format mp3 --audio-quality 5   --embed-thumbnail --add-metadata   "link to youtube video"  

If quality and file size matter to you, you should avoid re-encoding and stay with Youtube’s native music formats:

youtube-dl -f bestaudio[ext=m4a] --embed-thumbnail --add-metadata  "link to youtube video"

resulting in an m4a file

or

youtube-dl -f bestaudio --extract-audio --embed-thumbnail --add-metadata  "link to youtube video"

probably resulting in an ogg file.