A high floating salmon fly imitation when they are flying around and laying eggs in the rivers.  A must have fly for Hat Creek and the Upper Sac.  

Hitting the salmonfly hatch right takes a bit of luck, but if you do and you don't have a handful of adult patterns at the ready you won't ever forgive yourself. Do your future self a favor and pick up a couple now and stuff 'em in your box. Just in case...