My favorite ornament is not the big fancy flashy ones you would expect. But my favorite ornament is this really old generic ball ornament. This ornament is so old that most of it's color has worn off. Sure, I can find more ball ornaments anywhere, but this one has sentimental value, and lots of it.
Mine isn't one specific ornament, but a few that all came from the same place. My grandmother used to give out ornaments every year to all the grandchildren, and they were always little dogs, bunnies, or angels. All of us kids have one on the tree from around 1990-1992. I love them because they do have sentimental value and their old.
Having moved so much I cannot place my mind on one specific ornament as we have had so many commercialized stuff to fit our tree, but I so love what Konquererz put above and look forward to see my son do something similar one day.
When I was little my favorite ornaments were the glass ones. My parents also have an ornament with a picture of me as a baby that says "Baby's First Christmas" and I always loved putting that one on the tree! Also, like BabyBlues, my grandmother used to make us ornaments each year as well, so they definitely have sentimental value.