That’s a tough call.
Two of the best were:
* a moped when I was 14 (if I remember right). I told that story on my personal blog a while back and am not sure it is kosher to link here.
* a camera (specifically, a Pentax K-1000) a couple of years later, which gave me a lifelong photography bug. Not that I’m any good at it, but I still take photos (and still have the K-1000).
There was also the Christmas my dad gave me my first rifle, but I don’t want to stir the pot on HN…