The strength and usefulness of a knot is dependant on what the knot and line is to be used for. For example one of the strongest knots know is the "Palomar knot", this knot provides 95 percent of the lines rated strength and is commonly used by anglers to attach a fishing hook to its line. Its strength is derived from the fact that it is doubled up at the point where it is hoop-ed through the hooks eye. Of course this knot is of no use to someone who needs to join two pieces of line or rope together, in this case it is probably preferable to use a "Fisherman's knot", a knot know for its strength and resistance to slippage. If a strong loop is required, a "bowline knot" is probably best as this still provides 60 percent of the lines rated strength and and is not prone to slipping. The "Bowline with two turns knot" variation can provide additional resistance to slippage if needed.