Everyone has heard of the experiment in writing workshops where you give your first line and the others say if they'd continue to read on. There is one flaw that invalidates this experiment. It is done to other writers. Writers read differently than others, and most of our readers aren't writers. One of my most invaluable friends and beta readers isn't a writer. She is representative of my target audience.
One quote I keep near my computer and will constantly quote in my journal is "Craft is always secondary to the truth of emotional connection" Konstantin Stanislavsky. When writers read, they see flaws. When other readers read, they see pleasure. The most important thing I learned about writing, I didn't learn from a writer. I learned from a very modern major general. "People don't remember what you do or say (or write). They remember how you make them feel."
That all important first line is seen as a gateway to other writers. They will quote some of the great first lines of all time to tout its importance. Those lines are merely synecdoches, one line that stands for the gold of the rest of the book that made us feel something. That is why those lines are quoted.
The MOST important part of the book to a reader isn't the beginning. How many want-to-be-published writers spend all their time of the first line, first ten pages, first fifty because that is what will determine whether an agent will ask for more? Agents know this. How many agents are disappointed after they read that first line, first ten pages, first fifty because the writer didn't spend that amount of care on the rest of the book?
The most important part of the book to a reader is the end. It's the feeling you leave the reader with, the one that they carry with them. It is what determines whether they reread the book again to experience the feeling again. It is what determines if they pick up any of your other books because they trust you to give them a good story.