Well, I don't know if I have best birthday gift, per se. My birthday is very close to Christmas, and while my family has always been very careful to separate the two occasions, the gifts tend to blur in my mind. But I do have one very memorable birthday gift that I still have to this day.
On my 14th birthday, my parents gave me a Swiss Army knife. My father was a Boy Scout, and both of my parents had been counselors at a Y camp that I had attended for many years (a total of 12 before I was too old). So the knife was partially that "be prepared" philosophy. It was also a symbol, at least to me, of both responsibility and respect. It was not a weapon, but rather a tool that could be used in many different situations. When someone needed a package opened, a container pierced, or any other small task done, the person with the right tool could be there for them. So... that was my most memorable gift.