Always look to see what it says about use and washability and such.
I don't know if there is ever a perfect/best product, so many variables. I have used the old fashioned chalk, dressmaker chalk, other chalks. The texture of the fabric can make a big difference. Typically I prefer the lowest tech/cheapest item, but I will buy an expensive pencil or whatever is needed for me to see which is an increasing challenge.
I'm a fan of the Pounce Pad chalk with Full Line Stencils and recently got the Ultimate Marking Pencil. It worked great for half my recent project but then my dog ate it... she does that to chapstick and pencils and pretty much anything that drops.
https://fulllinestencil.com/ultimate...ng-pencil.html
The sliver of soap can be a great marker, just shave it down to the point you want.