What should I call this key?

Microsoft calls it the Windows key. Compiz calls it the Super key. freedesktop.org uses Win and Super interchangeably. GNOME ambiguously calls it Meta, Super, or an OS-logo shift key.

What should I call it when teaching others about Ubuntu?

For most users that's the Windows key since the popularity given by Microsoft after Windows 95 became a mainspread OS (look at this Wikipedia article).

For beginners keep calling it the Windows key but explain them that there are also another widespread name in Ubuntu for that key and it's the Super key, with this they become familiar with the terms.

In the other hand, for advanced users just use the terminology of Super key since they might already know that key with that name.

In that way all the new people getting into the Ubuntu world will became familiar with the terms of the shortcuts used in Unity and in other software and also it's easy for them to understand.

I would just call it the logo key. That way, people with a (possibly former) Windows computer look for a Windows logo. And the people with a laptop from something like Zareason or System76 that have an Ubuntu logo can still use that name, and when Ubuntu takes over the world we won't have to change the name.