My recommendations would be the 6x24 as everyone else has mentioned; a 12.5" square; a 3x6 or similar small size. My other recommendation is to buy all the same brand - whichever brand is right for you. When you start shopping, take the various colors of rulers and lay them over different color/print fabrics that appeal to you. See which color markings work best on the fabric colors/prints that appeal to you most. Then see if that brand has the various sizes you might need. I personally like clear/black lines...find those to be the easiest markings to see on most fabrics. If you can find a wider-than-6" x 24 and also meets the above suggestions, I would go with that.
Many times patterns call for 6.5" borders (or wider) and I love having a wider ruler to measure/cut with than butting 2 rulers together for those wider measurements.