The norm for 720s is yellow Kitty Regulars for most players
I use cotton on my wood set ups, and bulk poly on the bearing loopers. Fist Salud Loop lite is made by Kitty and its a special blend and between Kitty Slim and Regular thickness. I also use just YYE 100% bulk poly as well for economy. Lately I’ve been on a Red Slim Kitty kick just because I’ve had a bulk box for a couple of years and I’ve been feeling the color red.
Sting length is the most important variable. Too long and loops droop, too short and they get high and feel odd.
This 2020 video explains a lot about looping dynamics and set ups;
