I consider Omnigrid rulers first (there is both a 6.5 and 8.5 inch that is perfect for squaring up). This is primarily because it's a lot easier to use the yellow/black on different colored fabrics than a ruler with only black lines.
I LOVE EB's Flying Geese rulers. I have the large and regular set. While they are limited sizes, most of what I do fall into one of those four sizes. There is also now a mini set for miniatures. While there is a little more waste than the no-waste method, they are perfect every single time.