Any one thing from my collection? Thats a little tough.
I don't have anything outwardly valuble... maybe Starships of the Galaxy or Knights... but those really don't wow me.
Old stuff? Yeah, I have a couple books in my collection older than I am... autographed by their orriginal owners (who aren't anyone special by the way).
Best value? Well, I don't think my digital collection should count. Even though I got it all LEGALY.
If I were to have only one peice of my collection... I'd have to say I would keep ... The memories.
Books can be replaced, autographs are just ink stains, but the memories are worth more than my weight in gold (and I'm a BIG FAT GUY)