Although I read the whole collection it is the title story that intrigues me most with "In the Penal Colony" a close second. Kafka writes very strange tales. Tales that barely make sense on an intellectual level but work at a more instinctive and emotional level. "The Metamorphosis" could be a metaphor for a mental breakdown, but it is told in a much more literal way so that we can almost accept on face value that what Gregor experiences is real. It may have the best opening line of all short stories.
"When Gregor Samsa awoke from his troubled dreams one morning, he found that he had been transformed in his bed into an enormous bug".
Gregor is a devoted son who works at a job that he doesn't enjoy, for and with people who do not respect him, to repay a debt owed by his parents. It could easily be an essay on the dehumanising effect of work, and sadly when Gregor becomes no longer productive and useful he gets in the way of his family. A great and sad tale that should be on everyone's to read list.