I love my 8 1/2 by 24 1/2. Also use the 12 1/2 square and a 6 x 12 a lot.

One thing to remember is to be consistent with your ruler manufacturer on a project because the measurement markings can vary between manufacturers. Other than that use the rulers that fit with the style of quilts you make. I think I personally have about a dozen various sizes hanging on my wall.

I prefer a 6 1/2" to the 6" square I used to have. Also use my 6 x 12 lots, and 12 1/2" square for squaring up larger blocks.

I use a lot of strips for my quilts so my most used rulers are a 5'X24" Olipfa (love that edge), a 8 1/2" and a 12 1/2" square, and a 8 1/2" X 24" Omnigrid. I also use my 3 1/2", 4 1/2", 5 1/2", 6 1/2"and 9 1/2" squares frequently. Having the right rulers handy makes cutting and trimming much easier. I know I could probably do what I do with less rulers but I like to grab and cut without doing a bunch of figuring.

6 1/2" x 24"
3 1/2" x 9 1/2"
Shape-Cut Ruler with 1/2" increments
6 1/2" Square

In order of preference,

4 X 14
6.5 x 24
3 x 24

I love the 4 x 14 and have used it so much the markings and numbers wore off on the first one!

For me it's my:
6.5x12.5
6 1/2 inch square
12 1/2 inch square

And for my Dear Jane, my triangle ruler.

These are my staple rulers. I do have a few others though.

These are the ones that never seem to get put back in the ruler rack. 6.5 X 12.5, 8.5 X 24.5, and 9.5 square. Have many others "just in case".

Used with most quilty projects is a 6 1/2 x24",6 1/2 x 12 1/2" & 6 1/2" sq.Have several triangle rulers,a new 6 1/2" sq I hate :(,the flying geese set from QIAD....not opened & a 12 !/2" sq seldom used, as I'm not making many 12 1/2" blocks anymore.
There are others ,either never used or seldom used.But ,hey...I'm prepared for that next block :)

With all the patterns for Jelly Roll Quilts out there, I find I've fallen in love with strip quilting. I used to use a Square Up ruler and mark my line with masking tape. However, now that I am on my 6th strip quilt, I've treated myself to the Strip Tube ruler and I find I can cut strip pieces really fast and accurately with this ruler (You can probably use any triangle ruler, but I didn't have one of those). I love it!