I, too, have a 24" ruler that is my "go to", but it'w 5" wide. It sure came in handy when I was cutting 2.5" squares for the swap! I needed 15 squares of each fabric so I cut four 5" squares, then sub-cut each of these into fourths and had a square left over for myself!
I also have a 9.5" and 12.5" squares that I use a lot for squaring up blocks. I have a few specialty ones, too, such as Easy Angle and Companion Angle and Flying Geese, etc.
One that hasn't been mentioned that I love is my little 1" X 6" by Omnigrid. I have several of these (well, 4) and use them all the time. They are great for checking seam allowances, patch sizes (oh, that's why this block isn't fitting!), etc . I have one at the ironing board, one in my little notions basket next to my machine, one at the cutting table, and an extra. So handy!