#3 perle cotton. I also use yarn. I have quilts 20 years old and been washed many times and still holding together!!

simple quilter

Some great ideas, I've always use embroidery floss, all 6 strands. But the wool is a good idea, along with the fray check. I quite often tie off a quilt, and quite like the look, and is much easier than trying to stuff a queen size quilt through the throat of my machine, and cheaper than sending the quilt out to be custom long armed. Auntie B in Saskatchewan.

Perle cotton...it's strong and available in beautiful colors.

I prefer pearl cotton, but have also used crochet thread if it is about the same wait as the pearl cotton.

You don't look a day over 30!!!!

if i don't want to hide the pattern with yarn, i use knit cro sheen(comes on a large spool in quite a few colors and normally used to crochet). i have used it to make a large running stitch (1/2" long) or for tying.
i prefer sports yarn or a baby yarn to the thick 4 ply stuff! it lays flat and out of the way better. it also pulls thru easier & doesn't make such a large hole in the fabric, and you can use a smaller needle. much easier to work with.
remember if you use wool that you need to wash it in cold water and air dry.

Sounds like anything you can tie a knot in.
Robin in TX

If you have one of those old timey dime stores you might be lucky enough to find some "carpet warp". It is a tightly twisted cord that works well for tieing quilts and lots of other things that need a strong tieing material. It washes well. I found some among my mom's things and have used it quite a bit.