I use my 5.5" x 24" (I think that is the size--maybe it is 6") plus my 3"x18" the most.

I'm one of those quilters who loves rulers. I'm sure I have way too many. The one ruler that I use on a daily basis is Kimberly Einmo's EZ Jelly Roll Ruler. It is 5" x 10". It is great for pre cuts. I use my 6" x 24" ruler pretty often. I have 3 June Taylor rulers: Shape Cut Plus, Exact Eights, and Shape Cut Pro. I love those June Taylor rulers! They are such time savers! I also like rulers for squaring. I know you can get one square and use it for all sizes; but I like the exact size from 2 1/2" to 15 1/2". Of those, I use the 12 1/2" most.

I'm a Creative Grids ruler junkie. I have a bunch of them... and am planning on getting more.
For me, I got a set of squares 4 1/2, 6 1/2, 8 1/2, 10 1/2 and 12 1/2. I also got the yardstick which is fabulous for cutting width of fabric. I got a 4x12 as well. I got some triangles too..
I have several specialty ones for cutting curves, kaleidoscopes and whatnot and am planning to get the pineapple and the 45 degree ruler one.

I plan on selling all of my old rulers. After using Creative Grids, I want nothing to do with my old ones... and the ones that I have that aren't CG I lament the designer's choice of ruler companies lol.

I would say that every quilter probably has a list of ones they like the most. You'll have to figure out what you need along the way.. and even that changes. My theory is that if a ruler makes my life easier on making a quilt, I'm all for spending the money to do that. I don't need any more grey hair than I already have :D

I have the shape cut by June Taylor and I really like it also.

I use my 8 1/2 by 24 more than any others, since most of your strips or blocks have to be cut by the 1/2" example 1 1/2,2 1/2, etc.

I love my 5x24 inch Olipfa ruler. It hugs the edge of my cutting mat, ensuring that the ruler is perpendicular to the mat. . Straight cut every time.

I love my June Tailor Shape cut too, great for consistent sized cuts and I use my 6x24 a lot. I had a 2 1/2x 6 ruler that I used for small projects but Oliver ate it 2 days ago! His accomplice, Dave the cat, must have knocked it off my table and once anything hits the floor it seems it's fair game for Oliver. Of course I wasn't home and that's when Oliver and Miles give me pay-backs.

June Taylor shape cutter and a 6x24inch ruler.

My hands down favorite and mosted used ruler is.... 6.5 square. I have used it so much that I need to replace it because one cornor is no longer square but worn to be slightly "off". Second most is the old 6.5 by 24.

I have a Betty Gall 6.5 X24 that I bought in the 70's. I love that ruler, but the markings are wearing off. I don't like those thick yellow lines, but am unable to find one with nice thin black lines like the Betty Gall in that size. I do like the 3 X 9 Quilt Sense, but that brand is very hard to find.