If we were really organizing this page by how much we like characters, the Whale (oh, poor, sweet whale) and the Bowl of Petunias would be at the top of the page.
We love these two because (1) they make us laugh; and (2) they make us think. The whale and petunias scene really demonstrates how the narrative point of view works in Hitchhiker's Guide: we don't need to know what the whale and the petunias are thinking before they crash—that doesn't affect the plot and the other characters at all. It's pure and simple digression, but we get it anyway because this narrator loves digressions.
Also, the whale and petunias digression nicely shows off Adams's style of anti-climactic humor. We get this whole big description of the whale's thoughts and the way he innocently tries to deal with the world. Then, as the kicker to the set-up, we get two lines about the totally absurd thoughts of the petunia, who is apparently less innocent and more experienced than the whale (oh, poor, sweet whale). The climax to the scene is so ridiculous and unexplained that we can't help laughing.