It's interesting that, in it's simplicity, I find NeoAxis' engine demo the best yet for any engine. It has lots of rooms and in each he shows you a particular feature. It's renderer may not be as good as C4 and it's tools not as good as A7, but the developer (I think his name is Ivan) really puts a lot of effort in it.

It just comes through as a very honest piece of work.

Disclaimer: I don't own a license of any of the three engines above, so I am beyond suspicion.