Creative Grids! I'm fairly new and at my first quilting class the owner allowed my to try almost every brand on the market. Right away I found I didn't like frosted, or those neon yellow colored ones. True Cut was kinda cool, but expensive and I already had rotary cutters (their rulers/cutters work together).
As soon as I tried Creative Grids, I was sold. Easy to read, I love the 1/4" and 1/2" marks on either side, and best of all THEY DONT SLIP!!! So I have slowly been investing in different sizes. Started with 6 1/2 x 24 1/2. My other favorite sizes are the 2 1/2 x 12 1/2, the 3 1/2 x 18 1/2, the 6 1/2" square, and the 12 1/2" square. I just ordered their Stripology Slotted Ruler (it's on back-order) and am looking forward to that one!