1) What yarn (that you don’t have/haven’t used) would make your stash “complete”?
2) What yarn do you never want to be without?
For question #1, I would have to say one of the Tilli Tomas silk yarns like Plie or Pure and Simple. Even better would be to have a number of skeins of the silk with a couple of skeins of the coordinating beaded yarn. I typically don't stash yarns except for small projects. Typically I buy yarn for sweaters and I usually buy it for specific patterns. It is nice to have some basics on hand though for sweater patterns that you happen to come across unexpectedly.
This brings me to question #2. My favorite sweater yarns are Tahki Savoy (52/48 silk/merino), Berroco Pure Merino and Handmaiden Double Sea or Great Big Sea. Classic Elite Premier or Cascade Cotton Tencel are nice to have on hand for summer projects too. I have some Pure Merino and Premier in my stash but the Handmaiden is too pricey to stash so I buy it for projects that I am working on in the present.