The title is direct and self-evident, much like the story surrounding it. "We" infers a collective—a group of people defined by common terms—which is basically what the One State is all about. It stresses unity over individuality, and the actions of the many over the actions of the one. That lends it a certain sense of the ominous, as well as encapsulating the essence of the society it describes.
That, and D-503 specifically states why he "chose" it for his book:
I shall try to record only the things I see, the things I think, or, to be more exact, the things we think. Yes, 'we'; that is exactly what I mean, and We, therefore, shall be the title of my records. But this will only be a derivative of our life, of our mathematical, perfect life in the One State. (1.8)