Hmm, possibly the customised book my folks had made for my 3rd b'day (early 80s thing, ya could send away for one of five books, and they'd use your kid's name as the subject of the story), maybe the stuffed dog my grandpa gave me when i nearly died of pnuemonia when I was 4, or the transformer my first girlfriend gave me in grade 6.
Oh, also my grandpa's hat. He was a high school principal from 1940-something to 1960-something, and his last graduating class bought it for him as a retirement present. He wore it every day from that point on, taking care to clean it every night. My grandma gave it to me when he passed away.
Amazingly powerful, sentimental value is.