I have used regular yarn, wool yarn, beaver balls, cat whiskers and hush puppies. I liked the look of the soft wool craft store silencers (just about any color you want) and beaver balls if they're done right, but hush puppies wool is Much more durable and i think quieter. They're my top choice for wool.
With that said, those ugly old cat whiskers are weather proof and can be notably quieter than any of the other options. A few yard sales gave me a lifetime supply! I've seen some people carefully serve them onto the string and trim them down to an inch, and if you're gonna go that route, just use the bow hush wool. If i really want quiet i take the cat whiskers and tie them in a big knot on the string. Ain't pretty, but it works
I really like wool wraps on the end of my string on a recurve. The velcro idea seems great, i dont like sticking things to my bow.