I've bought most of my rulers in the course of doing Bonnie Hunter's yearly winter mystery quilt.
Easy Angle ruler - I use these for HSTs larger than 2 1/2" finished (smaller than that I use the free templates at Quilting and Whatnot), which I find much faster than drawing the lines
Companion Angle ruler - this is what I use to make QSTs
Deb Tucker's V Block ruler - I bought this because I did not get accurate results with the Tri-Recs tool
Deb Tucker's Wing Clipper - this makes 4-at-a-time trim down flying geese, I did not get accurate geese with the Easy Angle/Companion angle
The Simple Folded Corner ruler - this I use for fast snowballing
Creative Grids 60 degree triangle ruler - this makes cutting equilateral triangles quick and easy