It's probably a tie for what I use the most between my ShapeCut 12x12" ruler & my basic 3.5" x 18" ruler. The ShapeCut is great for cutting strips (and later on, squares/rectangles) that are all equal sizes. I have the worst time trying to line up my ruler to get properly sized cuts.
I'm currently in love with my 10.5" x 10.5" Omnigrip ruler because I made an Irish Chain with 10" finished applique blocks so that way I was able to quickly & easily square up all 12 blocks. It is also helpful for cutting nice squares since it has so many lines (45 degree, squares on the half inch, easy to read, all the lines are the same size). I like it better than the ShapeCut once I've picked up & moved my stacks of fabric because I know I'll never get them lined up perfectly ever again and my cuts will be off.