It depends largely, on what type of quilts interest you. I have a BUNCH of rulers but the ones I use the most are my
-June Tailor Shape Cut rulers (I have one in 1/2" increments and another in 1/4" increments)
http://www.amazon.com/June-Tailor-Sh.../dp/B000YZ7SJM, to cut strips and/or perfect squares
-two different sized 'square up' rulers, for trimming finished blocks
-small, square rulers with X through the middle, for centering designs
-several long rulers for various tasks
-several small rulers (6") for use at the machine
For the more expensive rulers, check out Hobby Lobby's weekly 40% off coupon.