Hand.Book Journal (or, my favorite art journal.)
From the delicious bookcloth cover to the best ever pages for my sometimes quite soggy style of mixing media, I am so in love with these journals! 130 gsm, acid free paper with just the right amount of tooth for watercolor and not so much as to be uncomfortable for simple pencil and ink sketching.
Tough and luxurious. Exactly what I want out of an art journal. My favorite art supply is likely my spray bottle full of water; it gets the most use, anyhow. And the pages in the hand.book watercolor journal can really stand up to my brand of creativity; I think I've damaged only one page in all the 4 years I've been using them.
A ribbon to mark your place, an elastic band to keep it closed, and a clear plastic pocket in the back to store ephemera; what's not to love?!
What awesome journal are you working in? Tell us why you love it in the comments below!