Your first choice as author (though it might not ultimately be your choice), is whether to have a start which makes your book as good a reading experience as possible or a start which sells the most books.

If Your Goal is Selling

If your main goal is selling your book, apparently you should write as interesting a first sentence as you can. Do not be overly concerned with such things as whether or not it is true, how much of your book it spoils, or how it distorts the telling of your story. Those are not your issues.

Then perhaps start with the most interesting scene in your book. If it occurs early in your story, you can flashback to the events before it. If it occurs late in the story, call it a prologue.

You also might want to prolong mystery, instead of filing in the context. You can intimate exciting events but describe them vaguely, so they don't spoil anything (or mean anything) but they do hint at exciting things to come.

And you don't have to worry about any of the problems and issues I talked about. Just write that great start!

Both Goals

Of course, you can (and probably should) have both goals – selling your book and writing a good book. The two goals don't always in conflict.

First, you should be trying to write an interesting start. Put another way, no matter where you start, you should be making an effort to have it be interesting. That's always good.

My guess is that the best choice here, if you can do it, is starting with action and close to the precipitating scene. That's honest, it doesn't spoil anything, and yet it tends to maximize interest.

Writing a Good Book

For writing a good book, I have suggests a plethora of factors that could go into choosing a good start. You are unlikely to have one best place to start your book. Instead, you have choices. There's a freedom in that.

But. You are not just telling a story, you are also constructing a story. When you have decide which event starts your story, you can then try to construct a good start.