I think I would like to try a knot quilt. What kind of batting should I use? thin or thick? cotton or poly??? Please advise!

The good thing about tying is that you can use thick batting with no problems. I've only used polyester for tying b/c cotton will get lumpy unless you tie every couple of inches (or whatever the label says)
I use perle cotton but others use a variety of things. I make a square knot and cut the ends to whatever length you wish (mine are fairly short)
One of my boys likes tied quilts more than any :)

When I first started quilting most people made tied quilts, it was either that or hand quilting. I got a lot of my scraps from a lady who had a cottage industry making tied quilts for sale. She had several women working for her.
Most people use thick batting but it is up to you.

I still tie quilts once in a while. I like the thick batting.

you should read the batting packages- there are some that tell you they are good for tied quilts- also you do not want one that requires quilting every 2"-4" that is pretty dense quilting and tying is not a method that will give you satisfactory results.

I tie lots of quilts. I use crochet cotton to tie them. I use extra loft poly batting...usually Fairfield

I love the look of puffed quits when they are tied. That's the ones I was raised with and what Grandma taught me. They are warm and comfortable to sleep under, and as couch throws they are marvelous, soft as clouds but heavy and warm. I love them.

I like a nice thick batting for a quilt to be tied. love the look! i have seen thinner quilts tied too. it's just personal preference.

I use bonded polyester quilt batting and have tied with several different kinds of threads (embroidery floss, crochet thread, yarn) Whatever the recipient preferred.

If I tie it is usually because I am looking for a certain look. So often my quilts that have ties were first quilted and I add the ties more as decorative.