1) What yarn (that you don't have/haven't used) would make your stash "complete"?
Um, I dunno. I've done linen, silk, LOTS of cotton. Maybe a cashmere blend of something. But that's one of those I'd pet and stare at and maybe never use.
2) What yarn do you never want to be without?
Cotton, but I'm more scared of running out of yarn, than a certain type. *Gasp* like that could ever happen. I'm finding all my yarns here at me mum's place, and that should keep me going for months still, with nothing else.