I don't have a lot of rulers. I do find them tempting when I see a new, fancy one in stores, but I've never yet bought one until I have a real need for it. Like after doing some crazy finegaling with my regular rulers to try to cut out perfect 60 triangles for a stack and whack hexie, I decided to use a 50% off coupon for the appropriate ruler.
So -- what are your essential, oft used rulers?
I have 3 that get very regular use: a 3.5 inch by 24 inch oldie that I bought in 1986. I love it because it's long enough to cut across a folded length of yardage. The 3.5 inch width is enough to give me a lot of options for most of the strip widths I'd want to cut. I love it so much the numbers are getting a little faint in places, which makes me sad, since I don't see these rulers around anymore.
a 6 by 12 inch -- use it lots for all kinds of chores. It's nice to have something more compact than the above ruler when I'm doing quick cuts. And it's big enough for most of my needs when working with fqs or pre-cuts
a 6 inch square -- use that for squaring up blocks and trimming units to size when needed