There's been lots of discussion on this topic. You may want to search for it here and on other forum's as well. However, I think the prime consideration is to find a string/thread that does not stretch and that has minimal or no slippage. Dental thread is commonly suggested.
There was also a link to a site that showed how to best tie the knot to avoid a bump when going round the pulley.
Bob
There was also a link to a site that showed how to best tie the knot to avoid a bump when going round the pulley.
Bob