I admit that I have many rulers that were mistakes and I dont use them.
The square in a square ruler by Jodi Barrows. Fons and Porters flying geese ruler and the triangle trimmers. Seams sew fast quide. Many others that I may use someday.
I use all my square rulers to square up squares. The 16 inch I use mostly to square up my quilts.
The rulers I use most are the 6 1/2 by 12 and 6 1/2 by 24, the Quilt in a Day flying geese ruler and the triangle square up ruler.
I also use the 1/2 inch Fons and Porter rulers so I have a line to follow since I cant seem to stitch a straight line.