MetaTube APP
The face that MetaTube shares an interface with the official YouTube app makes it surprisingly simple as you simply limit yourself to exploring the content as you would in the original app. Once you find the thing you want to save, just tap the arrow icon, set your preferred download quality, and tap Accept to start the process.
MetaTube has a downloads manager that lets you check the status of everything that's downloading in one glance. Plus if you only want the audio track from a video you can select MP3 as the output format.