I have been doing this for 4 years now, so I have accumulated many many rulers that I thought would be useful. Than being said the two I use regularly are a standard 6 x 24 and a 6 x 12.
My 6 x 12 is this ruler
http://www.olfa.com/RulersDetail.aspx?C=66&Id=144
I use it for all of my smaller trimming up projects because of the angle lines radiating from the center.