arkTube

arkTube is an app that lets you download videos from the official YouTube app. How? Simple. Just tap the share button while you're watching any video, and then select arkTube. You can then set the quality you want to download the video in.

One downside to using arkTube is that to download videos in high res you need to buy the donation app on Google Play. Otherwise, the highest resolution you can download the videos in is 480p. On the other hand, you'll find other interesting features like the possibility to download full playlists.

To sum it up, arkTube is a decent app that lets you download YouTube videos quickly and conveniently.