1) What yarn (that you don’t have/haven’t used) would make your stash “complete”?
My stash is a constant work in progress. Unfortunately I'm a collector (that's a nice term for it... perhaps "pack rat" is more appropriate) and building a yarn stash really taps into that. So the answer to the question is... my stash will never be "complete."
2) What yarn do you never want to be without?
A good basic 100% wool (or wool + other feltable fibers) that felts well. Brown Sheep Lamb's Pride. Cascade 220. Anything like that. And, of course, in a range of colors....