The experts say a first sentences must grab a reader and draw them into the story. There are all kinds of technical tricks you can (should) use. All manner of necessities.
But I think there is another angle a writer can consider.
The start of a book is the start of a relationship. So to me the most important thing about it is that it be genuine. Those first sentences tell a reader if the book is a kindred spirit or not. In some ways, the story itself matters less to a reader than the way it is told. You want to be using your real voice from the beginning. You want to have your favourite words right there to flare against, dissolve into, your reader's soul. Because we all like clever books - but we love books which get into our soul.
One way to think of it is that your first sentences could be like your smile. There's so much you can tell about a person from their smile. It's a language that speaks to instinct, to shared humanity. Writers are advised to start their story as close as possible to the moment of change, and technically that's good advice - but, from a different paradigm, I'd add that you should start as close as possible to the heart of your story.
Think of your protagonist, and smile. How does it feel? Is it happy? Tranquil? Sad? Tight and frustrated? Let the shape of that smile slide into the shape of words and onto your page.
Figure out what is the most important thing about your story, and show that in your first sentences.
It's advice I'd give anyone about life and relationships, really ...
Let your words and your way of being communicate from the start about your important things.
Never mind what the experts say about how you should technically manage those first meetings, what you should be wearing, how you should colour your hair or what car you should drive. My favourite writers break the technical rules but absolutely win my devotion because I love the heart and soul of their story. That story becomes a friend because of what it is on the inside. Same with my favourite people.
Today someone reminded me what is important to me. And I put that into my story. The relationship won't be overt to readers, but the spirit will be there. I think that's what it's all about. The spirit of relationship with a story, a person, an idea.
People love your books ... or your religion, or your values, or you ... when you open a space of belonging for them in that relationship too.