I think historians can get into trouble when they start prioritizing the most important events in our history. Everything is debatable in history, as is the importance of any single event. That being said, the Civil War is a fine candidate for such a description, if ever there was one in our history.
I call it a "watershed event" in my classes (said that very thing today, actually). That is to say, after the Civil War, the country had been permanently changed and headed in a brand new direction. It was a defining moment. It was when slavery of one kind ended (actual slavery), and slavery of a different kind began (sharecropping, tenant farming). It rapidly advanced our technology and concentrated wealth among industrialists. It expanded railroads, physically destroyed major swaths of the South.
Lastly, I would say it also began the point at which we moved away from loyalty towards our individual states and became more loyal towards our overall nation, whether that was the United States or the Confederacy, we no longer considered outselves merely "Virginians" or "New Yorkers". In my oinion, if it wasn't for the war itself, much of this wouldn't have happened, or wouldn't have happened until much later.