1) What yarn (that you don’t have/haven’t used) would make your stash “complete”?
That's a toughie. I don't think I'll ever consider the stash complete, to the point that nothing more could be added to it. There are yarns that I would love to have that would definitely enhance the stash, though! My Summer of Yarn Love pal sent me some lovely Noro, something that was on this list... I think other yarns would be Lornas Laces (fingering and worsted), Dream in Color (dk weight), Manos del Uruguay, and any Blue Sky Alpacas or Araucania that I haven't worked with yet! There's more, but that's what's in my head at the moment. Oh, also Tofutsies. I want to see what the fuss is about!
2) What yarn do you never want to be without?
A good worsted wool (Cascade 220, KP Wool of the Andes, etc), a good sock yarn (Araucania Ranco, KP Palette, Happy Feet, etc), and a good DK weight yarn (KP CotLin, etc). That covers most projects I would want to do! Specifics? I've always had Cascade 220 in the stash, as well as my handspun!