What is the Book of Mormon equivalent of John 3:16? Christians technically aren't supposed to have a "Canon within the canon" (i.e. a subset of Scripture that is somehow "more scriptural" then everything else) but in practical effect, we do.  If someone is interested in learning the Christian faith, nine times out of ten, someone will say, "Read the Book of John," if not actually just whip out John 3:16 and start quoting. (Very few, for example, would say 'start with Leviticus'. The truly erudite would say, "Read the Book of Romans," but I digress...)

So, I've read through Mormon exhibits that tell me about the journey of Lehi &amp; Nephi, of the battle that Helaman fought, and of the final burial of the Scriptures by Moroni - but what I want to know is this - what is the "core teaching" of the Book of Mormon that would be the most fair for me to evaluate? Not to discount the remainder of the LDS scriptures, but is there a "go-to text" that should be looked at first?

In other words, what is the abridged "canon within the canon" of the Book of Mormon?  Where do I begin when the guys in the white shirts say, "read this?"
If any such "unofficial canon within a canon" existed in the Book of Mormon, it would have to be in 3rd Nephi, from chapter 11 to approximately chapter 28.  This is from the record of Nephi, a distant descendant of the Nephi from the beginning of the book, who lived through a time period that our modern calendar counts as the 0030s AD: the time of Christ's mortal ministry, his death and resurrection.

This part of the record recounts how, at some point soon after the resurrection of Jesus Christ in Jerusalem, (the exact time frame is not given, but it is said to be "in the ending of" the same year,) the resurrected Lord appeared to the people of the Nephites.  He spent some time among them, teaching them about the principles of the Gospel and how the Law of Moses had been fulfilled in him, re-organizing the church among them in light of this fundamental shift in the law, and blessing and healing all who came unto him.

In fact, if "the guys in the white shirts" do come by and visit you, it's quite likely that 3rd Nephi chapter 11 will be one of the first things they invite you to read.  This section of the narrative is widely considered to be one of the most beautiful and spiritual parts of the Book of Mormon.