I have several, most are omnigrid. The 6X24 is the most used, but other are irreplaceable. The past few weeks, I've been using a triangle, a half triangle (I guess more like templates) and an 8.5" square.
I like the "nonslip" omnigrids, the ones that are green on the underside. When they are an option, I buy them. Otherwise the yellow ones with some non-slip tabs on the underside are the way to go. Nothing worse than cutting something and have the ruler slip out from underneath you!
M