Tubemate is a popular app to download videos from youtube and other various social video sites . It has both option play as well as download to your local storage . It provides various download in many formats and quality you can download full end 1280* 720 or 320* 240 quality videos in all formats like 3gp , mp4 also you can download in mp3 format .
It allows maximum 6 simultenous download and provides fast download options .but I notice some lack of features like many time viseo parsing stuck you will have to wait more than a minute or sometimes it failed. sign in to youtube also not working many times . Playing videos is not as fast as youtube .if you want to see full screen on mobile it takes some time to switch between small view to full screen view.
Also, app not available on play store you have to download from their partner sites .i found it somewhat useful but not best