We have rulers of all different sizes and manufacturers. Our favorite rulers are made by OLFA. The type we like are frosted and so much easier to see in use. I'm not a really big fan of the Omni-Grip line of rulers because they are really hard to read and they are really busy, marking wise. The nice thing about the frosted ones is that they are very easy to read against any color of fabric. The markings will wear off the Omni-Grip and render it useless after a time.
Size wise a 6.5" x 24" is a must have, you'll use it 4 times more than any other size. A 12.5" x 12.5" is a good second choice, but I prefer a 16.5" x 16.5" size for a square ruler. A 6.5" x 6.5" is nice to have but not really a must have unless you make a lot of 6.5" blocks. You can square a block with any square ruler.
Take a look at the OLFA Rulers here
http://www.olfa.com/RulersList.aspx?C=66