Some novels start with a bang, with a ingeniously crafted opening paragraph or sentence that instantly catches the interest of the reader and starts the concept of the novel. How important is it to have such a start?
I won't buy a book without one.
When I browse a bookstore, I look at all the titles on the spine. Only if the title intrigues me or implies a story that might interest me, do I pull the book out far enough to view the cover. I don't judge the cover image, but I use it and any text on it to get a first rough idea of what the book is about. If this interests me, I pull the book out completely and read the blurb. If this contains things that I enjoy and nothing I dislike (e.g. angels), I open the book to the first page and read it. If this incites my curiosity, I leaf through the book to read some dialog. If the author handles dialog convincingly (it is always the weakest spot in any narrative writing and the most irritating turn off), I sit down and read the beginning few pages and turn on my smartphone to read Amazon reviews. If everything is still kosher, I buy the book.
Today, there are too many awful books on the market and every reader has wasted too much of their scarce time on bad reading experiences, so your book has to overcome a series of filters before it gets bought. Because time and attention are scarce commodities for anyone with money to buy a book, they don't give an individual book much opportunity to hook them.
If you fail at the first paragraph, you fail.
It doesn't mean though that your first paragraph must deliver the Big Bang. Although many how-to-write books recommend this, a death in the first sentence or the fate of the whole universe at stake are usually a sure sign of a boring, generic book. You don't have to outdo all the other books ever written, just create an image that stimulates the imagination.
What makes a great beginning will depend on your genre and reader age. Here is what I think is a good example for a Middle Grade fantasy (from Ursula K. Le Guin, A Wizard of Earthsea):
The island of Gont, a single mountain that lifts its peak a mile above the storm-racked Northeast Sea, is a land famous for wizards.
Books today no longer begin "once upon a time..." there was such and such a hero, and such and such a heroine. So you do not want mostly "background" in your opening paragraph.
Instead, I was taught to "begin the story in the middle of the action." An example is the opening of my Revolutionary War novel.
"Outside a log cabin on the banks of the Pee Dee River, Charlotte Mouton saw a group of horseman riding in the distance toward her. They weren't wearing uniforms so they had to be American Patriots, not British Redcoats, or Tories, pro-British Americans who wore green. She wondered why they were headed north. The enemy was not here, but further south. And according to the rumors that had been spreading through the countryside like wildfire, these men had been winning their battles. That was saying a lot for a bunch of farmers whose patriotism and enthusiasm for war far exceeded their training and experience.
The year was 1780, and the American Revolution had been raging for four years. But the fighting had only recently come to South Carolina..."
Under the "once upon a time" format, the most recent paragraph would have been the opening paragraph. But you can see how my "action" paragraph is more compelling. This becomes even more so, shortly, when we meet Charlotte's older brother and her future husband among these American soldiers.
Quote from Chekhov: “It seems to me that when you write a short story, you have to cut off both the beginning and the end. We writers do most of our lying in those spaces. You must write shorter, to make it as short as possible.”
If you read most of Chekhov's stories, you'll probably find this is true. There's an essay that can answer all your questions called 'Overtures', written by Simon Leys, if you can find his book called the Halls of Uselessness. He writes how some authors, like Anthony Burgess in Earthly Powers, can write the most interesting things at first but completely fail to deliver. On the other hand some writers validate their first paragraphs like Melville in Moby Dick.
William Gass said that a better idea of finding out whether a book is good is flipping to pg 99. So I don't like the idea that it has to be the first paragraph that grips because some writers write in such a way where the foundations they stack very early on are almost imperceptible but when you reach the end it hits you like a trainwreck. I don't think people consider the opening of Madame Bovary to be very interesting, but Flaubert poured his whole self and years into perfecting that novel. Likewise some people won't be gripped by the opening of Proust's massive monster book, but its still a great classic of Literature, Time, Memory and Sensation. Somehow it works the same for endings. A story can end quietly but it can leave a sting that grows over time. Chekhov's The Lady and the Dog is sort of like this, and Kafka's famous The Castle doesn't even have an ending.