I don't like the ones with the yellow over the markings, like Omnigrid.
I have a Betty Gall 24 X 6.5 ruler that I really love. Just have to make sure I know it is 6.5, not 6 inch. But the markings wore off. I can still use it if I use the markings on the mat. That ruler is 30 or more years old. My first love!
The one I really like is Quilt Sense. I have a 3 X 9 inch, which is soooo handy when trimming small things. Has very thin black lines. I haven't seen these in the stores much.
My Optima 12.5 square has thin black lines on one side, and thin yellow lines on the back. I love that one. Useful on light or dark fabrics.
My friend has one with thin white lines. I would like that if I bought another one. For a while, I bought a new ruler at each class; not necessarily needed for that class, but one I thought I would use. My latest is Creative Grids 60º Equilateral Triangle by Rachel Cross. Has alternate black and white thin lines. Works very well on my curent project: cutting triangles.