What is the name of the company that designs planets in Hitchiker's Guide to the Galaxy, or does it ever say?
The one which Slartibartfast works for.
I don't think we are ever given a 'company' name.
The planet that is the home to "custom-made luxury planet building" is called Magrathea. However we are not told how it is organized economically. It becomes the richest planet in the galaxy, but it may be that there are several corporations that operate from it, or none.