I don't know that I have a favorite tool, but my 20-1/2" square ruler is certainly one of my favorites. It is incredibly useful when I have to lay out and cut large pieces of fabric; most of the fabric I buy is large pieces. At the same time, I use something I created: large-ish plastic buckets of smooth decorative rocks that I use as weights to keep the fabric secure when I line it up and smooth it out. I also love my homemade shelf units that are the "legs" for my large cutting table.
The most useless?? I have MANY things I've bought over the years that I have not yet used, but the thing that concerns me most is EQ5. I was reluctant to buy any quilt software, but decided if I had an opportunity to buy EQ5 at a reasonable price that I would buy it. I did buy it, but it holds no interest for me. I played with it for a few hours each night when I first got it, but it bored me to tears.