I'm a big fan of the Prussic. It's insanely useful while camping, and for other things as well. Bowline, Square knot, figure-8, lark's head and a fisherman's knot are also useful.

All my guy lines are a small closed loop attached to the tie out point, and then prussic-ed to a line with a bowline at the end. It means I can quickly adjust the tension from inside the tent, or tarp. If the weather is really awful and the rain nearly horizontal I can stake through the closed loop and really batten down the hatches.