Definitely the rotary cutter, ruler and cutting mat combo. I know that my hands/wrists couldn't handle cutting much fabric with out those tools, so I wouldn't be quilting if they weren't available. Besides that, they save an awful lot of time!
My new favorite is the square grid ruler, every block lines up!
I've been thinking about this question, and it's been hard to think which notion or tool is my favorite. Since I enjoy applique - I would say my applique #11 straw needles and silk threads, my awl and small Clover iron. But then, I couldn't live without Omnigrid multi surface to iron and cut, or the 1/4" clover bias maker, or "quilter's crack" - Best Press starch or freezer paper. But ultimately, it's got to be rotary cutter/rulers/self sealing mat. Lots of time savers out there like the Accucut Go/Go Baby, but I really like good sewing thread and new sewing machine topstitch needle 80/12. Soooo many good notions!! Just hard to choose!!
There are so many I couldn't live without, but my favorite is my thimble. I bought a nice open top one from Roxanne. I hate hand sewing without it.
I can't pick a favorite, but I'm enjoying my new Bernina stitch regulator. Couldn't quilt without my Machingers quilting gloves. Love my Grabbit magnetic pincushion, too, because I can throw the pins at it as I sew and it catches them every time!
I think Best Press is a must have. I also love my rotating cutting mat, and I use my Purple Thang a lot!
I love me some Best Press and really love my Clover silicone thimble.