I have about 3 dozen rulers and my favorites are the Omnigrip and Omnigrid (I add the non-slip film that Cathe mentioned).
I think the rulers I use the most are 6"x24", 4"x14", 12 1/2" square and 6 1/2" square. I also love my fussy cut rulers.
The small square rulers (2 1/2 square and 3 1/2 square), I use very seldom, because they slip very easily.
I'll also add that you don't need a lot of rulers to be a good quilter. I just can't resist them. :lol: