I assume "best performance" means distance in this case.  In my collection of slings, in terms of power, nothing comes close to the kelvar ones I have made:

http://www.slinging.org/forum2/yabb/YaBB.cgi?board=1;action=display;num=11011010...

I think they look cool as ice. 8)

I find braided slings in general are too thick and heavy for serious distance, but they have better feel and may be more accurate.  Also, thicker cords tends to tangle less often.  Longer slings have more power but I find shorter slings easier to control.  Short slings has the advantage of a faster wind up too.  So it is really a trade off.......

The colourful ones are braided with nylon and the primitive looking ones are braded with hemp.

You can get Kevlar at a juggling store?

The ones I got is a tennis string.

I don't know  ???.  I didn't buy them.

yeah, i looked around my house and found some 'masons line' and i'm making a simple braided out of that, but is nylon the best for making one?

I'm fairly new to slinging, but I made my sling by buying a pair of bootlaces and tieing the ends of the laces to the lower areas of the other lace. The pouch doesn't have a real bottom, and it's very thin, but it's flexible, durable, and you can adjust the pouch size. I've slung small rocks, baseballs, softballs, tennis balls, golf balls, and even tried a basketball though it didn't really go very far.    Well anyway, later instead of just keeping it like that when I saw how torn up it was getting when I slung jagged rocks, which are what I have the most access too, I sewed the strings to a peice of leather and I have a very stable sling, that's slightly longer than my armspan, and can sling projectiles comfortably that are about the size of golfballs or smaller.

It's not nearly as cool as slings made using nicely colored fabrics, and it's thin, which results in some tangling, and can only sling pretty small loads, but it's economic, reliable, and adaptible.

Unfortunately, there is no "better".  It is like comparing a road bike and a mountian bike, each has it's advantages.  I find it easier to make the leather pouch with cords.  That's what I started with.  Eventually, you'd want to try everything and find out for yourself which ones are "better" for you.

This might just be my opinion but just make do with whatever you have. It's fun to experement with new materials and designs but for me the gain in distance from using superthin lines and such isn't that big a deal. For me it's like adding another 25 metres. So instead of 275 it's like a little more that 300 metres. And if you can hit something at that distance then I am super impressed. (I sometimes hit seagulls but I aim at a big pack of them)