Well my friend, that depends. Slinging is a hobby of mine so I will help where I can.
If you want the best then use 550 paracord and weave one. as long as what you're lobbing is under 39 stone in weight you will have REAL difficulty breaking the cord
If you want traditional AND strong then use fibrous plant material (nettle stems are easiest in the uk) and, again, weave one. If you want traditional and quickly made then,,,,,you know what,,, here's a link to save me typing.
http://slinging.org/forum/YaBB.pl
Hope this helps,,, Oh, coincidently, I had my stingray box out and there is a photo of my paracod sling attached now.