Douglas Adams is funny in a very British way and I like Brit humor. So I found both the book and the movie hilarious. If you ask, however, which I liked better, I would have to say the books.
The Ultimate Guide is said to be a “trilogy” but it contains 5 books. All were good except probably for the last one named Mostly Harmless which got a bit strange in an un-funny sort of way.
The movie was also good. I am not a purist about such things, so I had no problems with the differences between the radio show, books and the movie. Plus hitchhiker’s guide already has lots of versions. But there are a couple of saccharine movie moments which are very unlike the spirit of the radio show or the books. Also, the two heads of Zaphod Beeblebrox were not handled very well in the movie. I would have preferred the two heads to be visible together. I would rate the movie an 8/10.