Never watched "Hitchhiker's Guide to the Galaxy", thanks.
Read the book first ... I feel the series did do it some justice, but the book is awesome!
Although, if you can find it, the original radio series was probably the best interpretation...
Technically it doesn't really qualify as an interpretation. The original radio series came first.
One of the fun things about it is that every version is different. The books are generally held as the definitive version because they are easiest to get hold of, but all of the interpretations are good. This is primarily because Douglas Adams was involved in writing all of them (even the parts of the 2005 movie version that weren't in any other version came from some of Adams's notes, such as the things on Vogsphere that came out of the ground and smacked anyone who had an original thought around the face, explaining the Vogons' physignomy as well as their unimaginitive, bureaucratic nature).
I recommend the first two books, which basically cover the essential story. The third is an adaptation of a Doctor Who script Adams was working on but was not used. The fourth tells a whole different story, and the fifth, well...