TubeMate

Tubemate
Tubemate is an app to download videos from Youtube directly to your mobile device. Using the application you can save your favorite videos directly to your SD card.

When accessing Tubemate, you will notice that the app's interface is actually a mobile tab of Youtube itself. So you use the app as if you were directly on the Google video site.

Finding the desired content, just start its playback, as soon as the video window starts loading, a download button will be available.

Tubemate allows the user, before downloading, to define a quality and format of the file to be downloaded. By default, downloads are saved to your SD card, but once completed, files can be moved.

If you log in to YouTube through the Tubemate app, all your favorite history and even recommended videos are exactly the same as the ones you use directly on the official YouTube website or app.

also check out the SnapTube app.