This is almost a "use" question. I don't know any quilters who have only 1 ruler. That's like asking, do you like fabric? These days, I really like the ones that have the little grippers on the back so that they don't slip. My most favorite ruler when I am squaring up a quilt is an 8.5 x 24 that I have. When I am doing normal cutting, I have a 6x24 that is good. When I am squaring blocks, I have a 6x6 and a 12.5x12.5 that are my favorites. When I am doing embroidery, I have a ruler that is 15x15 and has slots to mark where I want to center the embroidery pattern.
Seriously, most of the longer strip rulers 4.5 x 24 or 6.5 x 24 have all the lines to do triangles and 60 degree items. You can make do with one of them, but you get to work a bit harder to do it. When buying a new ruler, make sure you can see the lines on both light, medium and dark/black fabric.
Good luck!