I only have a few rulers - almost all are Omnigrid. I'd say this is the current frequency of use order:
1) 12.5" square - almost every quilt
2) 6.5" x 24" - almost every quilt, esp. for borders/bindings
3) 6.5" square (my newest purchase)
4) 3" x 18" (the first ruler I had) - when I'm cutting strips more than blocks for a quilt
5) Judy Hopkins' Scrap Saver (more of a template than a ruler but it gets used more as my scraps get smaller and need to be turned into triangles)