Whichever rulers you decide to get, I highly recommend that you get all the same manufacturer. They *play* better together. Seriously.
While at the stores, take two of the squares to some fabric and lay it out and use the tools to pretend to cut. Do the tools make sense to you? Can you easily see the hash marks and the intersects? Try them on a few different fabrics.
My personal preference is Creative Grids. When I started quilting I started with a 6X24, a 6.5 square and a 12.5" square of two different manufacturers. At some point I needed a different size and I bought a Creative Grids. I have since collected every size they make. They are well marked, can be used to the left or the right, easy to see, have 1/4 and 1/5 edges that are frosted, have non-slip dots on the back, marked angles with good intersecting hash marks. All around - great rulers.