Polyester batting works best for tying. Most people use the medium weight. There's basically thick (18 oz or 9 oz), medium (12 oz or 6 oz) and thin (8 oz or 4 oz) polyester batting. The way they name it is if you cut off a yard and weigh it, how much it weighs. So if the batting is 96" wide, and 1 yard weighs 12 oz., it's a 12 oz batting, which is a medium weight. The same thickness in 48" wide would be a 6 oz batting. The extra wide 120" batting comes in a medium weight, and is named 15 oz batting.
A great way to mark quilts for tying is to use the Cambridge Marking System. It's like an interfacing with circles cut out, in a grid. You lay it on top of the quilt and mark the spots with a chalk pencil or pin or whatever will wash out.