best app to download youtube videos to watch offline?

The legendary file manager Goodreader has that ability built in. Essentially, when you play the a YouTube video in Safari, you can tap a bookmarklet and it will import the video into the app. It works for all HTML5 videos,including YouTube ones.

YouTube Red allows you to do that, among other things. If you watch a lot of YouTube, it's worth the money

I use amerigo for all my downloading needs on my iPad mini.

I use Workflow to do this without JB.
https://www.reddit.com/r/workflow/comments/2p0vga/workflow_download_youtube_videos/