I get my rulers when JoAnns has them half price.
My favorite work horse ruler is the 8 1/2 x 24. After that my 6 1/2 inch ruler and 12 1/2 square up get used the most. I do a lot of quilts on point and have one to cut the triangles on the edge. I like having the straight of grain on the ends so I don't like cutting squares in half for the set in triangles. I like cutting them from a strip.
I love any ruler that will save me time. But some I can mark off on the rulers I already have.