Betonart Audio Arrivato Speakers...Cabinets made of Cast Concrete!..

Is it the best?   No 

It's just a design option that has to be balanced and integrated with the total speaker design. Did this mfg'er do a good job integrating and optimizing the design? I won't know unless I hear them.

I don't remember the name but back in the day (over 20 years ago) there were others who used cast concrete. I don't think they're still around though that doesn't validate or invalidate the concept.

There're a few speaker manufacturers who use stone or concrete of some sort:
http://andersonspeakers.co.uk/
http://www.norh.com/Norh_Loudspeakers/Norh.html
And vintage Gemma speakers from the 80's were made from marble

And I believe there's another one with two last names as the brand - like "__________ & ___________", but all I can think of is Harry & David, which isn't it.

There have been a few. I own the Celestion Kingstons that are made of a man made synthetic stone that is like concrete, and it does free up the sound from the enclosures. 

I heard the Acora Acoustics granite speakers and was pretty impressed.