When making a loop for a Prussik sling, a double or triple fisherman's is preferred. I find supple 5mm cord to be the best balance of strength and grippyness for most ropes.

Just to confuse the issue, there are other friction knots that can be used for ascending a rope including the Bachman knot and the kleimheist knot. Both are especially useful if you do not have cord handy as these knots will work with tubular webbing and sewn slings.