I think so far, it's been the stuffed two headed mynah bird from my partner. :) Also a close second is my Nook, which was a joint gift from my partner and my dad.
A family genealogy book published at the end of the 1800s that traces back to the 1600s. It was thought to be lost after my grandmother passed away; but her sister gave it to one of my aunts, who thought I might like it. Like it? Every time we visited my grandparents I'd pour over the pull out pedigree; I'd read the short biographies of some of the ancestors. I've never been so touched as when it was given to me. In fact, it's sitting in my bookcase, immediately to my left, right at this moment.
A book I'd browsed once in a library 25 years earlier, but never forgot. I mentioned it in a Metachat thread a few years ago. A Mechazan across the world recognised the book (which was a pretty obscure book), located a copy in England (amazing, considering it couldn't remotely be considered a best-seller) and had it delivered to me, anonymously.
I found out later who it was, and I think of them every day when I see that book on my shelf. I will never forget the feeling of utter astonishment when I opened the package and that long-lost book was in it.
Like Miko, mine is the viola my family clubbed together to buy me for my 18th birthday (I chose it) and which I still play 20 years later. It has grown and improved with me.
Probably the times when my husband gave me the gift of an entire day to myself when my daughter was young. I get very drained by constantly being around people, and when you have a little one who really really wants you all. the. time. (she was a real mama's baby, and very very cuddly, which was awesome, but also very difficult for me, who is not a touchy person, even with my own kid) it can take a lot out of you. I often got to the point where there was just nothing left of me, or where I felt that if I had anything left to give, my husband took it (note: this was unfair, but those were my feelings). So, an entire day to myself to do exactly what I wanted was perfection. I would get up early and leave before anyone woke up, and wouldn't come home until very late.