Wow, tough questions, like last week. I'd have to say that the yarn that would make my stash compete is a yarn I saw a year ago at a small novelty shop. It was this silk yarn that just flowed like water. At the time I could neither afford it, nor would I have been in the mood to make something out of it, so I talked myself out of buying it.
But I still think about what I would make out of it. I suppose it comes close to a yarn I dream about. I'm sitting on the beach, and I grab the water and twist, and it becomes yarn, which I begin to crochet. As I crochet, I make a blanket, which if you look at out of the corner of your eye, contains the ocean- fish, mermaids, sea-wrecks, whales, dolphins, squids, coral, and all manner of other creatures. It is a fey blanket, destined for someone that cannot bear to be without the sea, but must.
The yarn I saw in the store was a very pale approximation of that dream yarn, but it is the closest I've come.
As for what I can't be without? Some cheep acrilic for when I need to make something simple or easy, or I want to test out a pattern I'm working on. Some wool, because I love the feel of it through my fingers. Bright mohair, and some nice alpaca, because again, I love the feel. A solid, affordable cotton for when I'm working on cheep therapy- washcloths.