I rarely use rulers. I do mostly freehand and custom work with very few pantographs.
I use rulers for stitching in the ditch. I have two that I use most often, the one that came with my A-1 Elite longarm machine and a nice little Megan Best ruler. I love the little ruler because it fits my hand so well, and also because it has a quarter inch cutout along the side. That allows me to hold the ruler on the seamline, while the machine stitches in the ditch. You can get it at
http://www.bestquilter.com/2.html
When I want to make straight lines (not stitching in the ditch), I prefer not to use a ruler at all. I prefer MARKING the straight lines on the top with chalk, either by drawing with a piece of school chalk or by pouncing with chalk.
I find the stencil by Pam Clarke, with straight lines one inch apart, is one of the most useful tools in my workroom. I use it for piano key and beadboard borders and also for marking small spaces for crosshatching, like behind applique.
http://www.homestitches.com
I'm not affiliated with these women or their companies.