UmmyVideoDownloader works for me. Free, fast, dead simple, works a treat. You can download the Youtube video in MP4, or just the audio in MP3. Then you have the editable files to play around with if you wish to create longer videos**, or playlists in MP3.
I find Filmura the easiest and best for creating/editing videos, but there may be better. It can add music and voiceover tracks to the original video and audio. It's quite intuitive.