OK - those are strong words, and there really has been a lot of great writing in the last 50 years (I did say "possibly"). But the combination of imaginative genius, over-the-top humor and just enough philosophical thought makes H2G2 a true standout. The radio plays predated the published books, and were my first exposure to Douglas Adams in the early 1980s - they remain the best way to experience The Hitchhiker's Guide.
There are only a few audio books that I return to and listen a second time. H2G2 is an audio book that I am moved to listen to at least every two years. Basically, I'd be lost without the Guide... If I could give it six stars, I would.
P.S. Years after I fell in love with the books, I learned that Adams had been a script writer and editor for the BBC's Doctor Who, and that he was a friend of Richard Dawkins - he introduced Lalla Ward (who played Romana on Doctor Who) to Dawkins, they subsequently married. Lalla Ward co-narrates Dawkins' audio books, which I also recommend. That serves to illustrate "the interconnectedness of all all things" (which is something you'll come upon in another of Adams' books - Dirk Gently's Holistic Detective Agency.