If I may clarify the intention of the original post:
I believe the thread submitter was looking for a single, physical possession (not an abstract concept or living thing, and not something general like "my clothes" or "computer gear"). That being said, the physical possession does *not* have to be the possession of highest monetary value. It can be something with sentimental or other non-monetary value. Submitter, did I get that right?
So by the above criteria, my most valuable possession, sadly enough, is my MacBook. And not just in monetary terms, but also in terms of the utility I get out of it every day.
I don't own a house or a car (living in NYC and all), and none of my clothing or furniture or electronics is more expensive than my MacBook. I have a suit that I think was close to the price of the MacBook, but since this is a Mac forum, I'll give the prize to the MacBook

(and anyway I never wear the suit so I don't really value it).
Edit: and I don't think financial investments should count either. If I included financial investments, my single largest financial holding (and largest "possession", if we include it) I think is an S&P 500 mutual fund. But I don't think it should count.