I'm pretty sure my favorite item in my collection is my original copy of Earthbound. (I know, not much of a surprise.) It was the first game I ever purchased without renting it first. I had read so many reviews that I felt comfortable spending the money, and the local Meijer (basically a Midwest Wal-Mart) had it for $30.
My mom kept asking me if I was sure, and I finally turned to her and said, "Mom, I've read so much about this game that I can probably already finish it in my sleep." I would have been 12 or 13, and Earthbound was the first RPG I ever actually played. Years later, it's still my favorite game and I'm proud to say that I have been the only owner of my personal copy.