I use all sorts of cordage. The stuff I've used the most of in recent years is for my many possum snares. I buy it on spools from a local commercial fishing supply company. It is a simple braid with no noticable inner core to worry about. All sizes of the stuff cost about the same per spool, but of course you get a longer length of the thinner stuff on a spool. I'd pay maybe NZ$25 for a 1 kg spool, and this gives me an awful lot of string.
Here's a link to my favourite rope shop:
http://musselrope.co.nz/
And here is the stuff I'd typically use for snares and odd jobs:
http://musselrope.co.nz/Nylon_Flat_Braid.php
I've grown up in a family that has boats and machinery. So I've had a long familiarity with things requiring cordage. Fishing lines, nets, complete chain and rope swing moorings, general boat ropes, tow ropes, log hauling, tying down loads, vehicle rescue, tethering animals and makeshift repairs.
I've played around making my own natural cordage. And, like primitive firelighting has made me appreciate matches, (even though I can 'make do' in the wild) it has given me a huge appreciation of the strength, stretch, durability and relative low cost of synthetic fiber cordage.
Natural cordage snare for small animals (untested):
Mooring ropes (polypropylene and polyester here I think):
Braided cord snares. The black one has been impregnated with some kind of 'tar' --- presumably to give it certain better qualities for some fishing application. I've caught dozens of animals using black cord like this.