The official one works great for me.

Why not use the official one or the Google cast feature from the phone app?

From what I've bumped into, there are 3 main options:

1. youtube official app (which works fine unless you're trying to read comments)

2. kodi

3. sideload chrome, and browse to youtube.com. If you set bookmarks on your chrome they should appear on your Leanback home as well.

To solve the multi user issue in our home I installed full android on my shield (there's several decent roms on xda), set up two profiles with different Google accounts for different YouTube subscriptions etc. and created one click shortcuts to switch user using Tasker.