This is super tough for me, there are so many items I simply couldn't work without.
Though I'm not particularly fussy when it comes to crochet hooks, honestly any old hook works fine for me, I am more so when it comes to knitting needles, I haven't tried them all and certainly have yet to work with some of the best, but some of my current favourites are knitpicks interchangeables (they aren't perfect, they do come unscrewed regularly and I have snapped more than one, but I do like them) my other favourites are a lovely pair of square rosewood needles I purchased on holiday one year in a really sweet knitting shop in Historic Georgetown, South Carolina. I so rarely knit on these as they are really chunky, but there is something about square needles that seems so right!
However after a chat on the subject for crochet hooks over on twitter a couple of weeks ago, I have been convinced to try out some of the better ones. Top recommendations were for Clover, both Amour and Soft Touch, though if anyone has any further suggestions I'd love to hear them.
If I really had to choose my most coveted tool, I guess I'd go for my books, I have quite a collection. I literally could not design without these, my stitch dictionaries are invaluable (Kat Goldin did a fab post on this exact subject a while back which saw me adding a few more staples to my collection!) but also Elizabeth Zimmerman's 'Knitting without Tears ' and Debbie Abrahams 'Design Your Own Knits in 5 Easy Steps ' have been fundamental to assisting with my designing.
Currently my favourite stitch dictionaries are "Harmony" Guide to Crochet Stitches and Complete Book of Crochet Stitch Designs
I'm also currently awaiting delivery of 63 Crochet Cable Stitches.