I have a LOT of rulers and templates.
The ones I use the most are
6 1/2 x 24
12 (or 12 1/2 I can't remember what it is) square ruler for squaring blocks
5 square ruler for squaring smaller blocks - can do it with the bigger one, but its less cumbersome with the smaller one.
EZ bias binding ruler
binding tool (more of a tool/template I guess but I keep it with my rulers)