One of the first rulers I bought was the Quilter's Rule with the ridges on the back.
What I don't like about it now - the lines are "thick" - and I like the rulers with "thinner" lines now.
Creative grids has an 8.5 x 24.5 ruler - it's nice, but I do need to pay attention to the markings on it.
I think the 6.5 x 24 ruler would probably be the most useful - if one could only have one.
The next choice - if I could only have one - would be a 6.5 x 12 ruler.
For small cuts, I love my 4 x 8 Omnigrid ruler with the 1/8 inch markings.