"What !? Windows 10 doesn't have a official YouTube app?" This is something I have heard from my friends, specially in the case of Windows 10 Mobile. For the record, Windows (phone) never had a official YouTube app. Thank Google for that. But that doesn't mean that there is no way to access YouTube. You can do so using your web browser to access the mobile site for occasional browsing and watching videos. This won't suffice if you want save videos offline, get notification from your favorite channels and other advanced features which you can find in the official YouTube app in other platforms.
Fortunately, there are several beautiful third party apps that are equivalent to the official apps. Here is a list of such apps.
1) Tubecast
Tubecast is one of the finest and full featured YouTube client that you will ever find in Windows. The interface is clean and slick. You can log in using your Google account and bring your favorite channels at the finger tips. You can also comment in a video without leaving the screen and the app supports swipe gestures making navigation a breeze in touch enabled devices. Offline videos (Preload) are supported and you can choose between SQ,HQ or HD for saving that video. It has a live tile and supports push notifications to keep you updated.