The first two are fantastic in the literal and colloquial senses. The remainder are only weak in comparison to what went before. So many wonderful concepts and phrases:
Vogon ships "hung in the sky in exactly the way bricks don't"; the Hooloovoo; the old man who said nothing was true but was later found to be lying; spend a year dead for tax reasons; meat bred to want to be eaten; the "knack" of learning to fly is to "throw yourself at the ground and miss" (I think Buzz Lightyear borrowed that); "Aggressively uninterested"; Slartibartfast; "One of the least benightedly unintelligent organic lifeforms it has been my profound lack of pleasure not be able to avoid meeting" (I think Boris adapted that one); "it makes as much sense as the sea being parallel"; the future is "just the same old stuff in faster cars and smellier air"; "the sun was quite bright but the day was hazy and vague".
Brief summary and favourite quotes from each of the five books, as follows:
Hitchhiker's Guide (vol 1): http://www.goodreads.com/review/show/107767859
Restaurant at the End of Universe (vol 2): http://www.goodreads.com/review/show/107768059
Life, the Universe and Everything (vol 3):
So Long, and Thanks for all the Fish (vol 4): http://www.goodreads.com/review/show/107768203
Mostly Harmless (vol 5): http://www.goodreads.com/review/show/107768298