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. 

Mark and Daniel! That was it. They make speakers out of what's essentially formica and CAM process granite.

Cast concrete technology has come so far that Wilson and Focal were both looking into it. And, rumor has it, Revel and B&W. But they all dropped the idea upon learning the speaker cabinets would have to be 100 inches thick in order to meet the approval of the Kenjit Society.