Fifty feet of 3 mm braided nylon. This is the gauge for anything that does not need to hold weight: replacement guy lines, a ridge line for drying gear, lashing a tarp corner, hanging a lantern.

It takes a knot well and does not stiffen up when it gets wet and cold. Cut what you need and melt the end so it does not fray.

If you are tying down anything heavy, step up to the 5 mm or 7 mm.