1. Reef knot to join two ropes together.
2. Bowline to fasten a rope to a strong point.
3. Clove hitch to fasten a rope to a pole (or such like) quickly and you can easily lengthen or shorten the rope.
4. Figure of eight and on the bite used as a stopper knot (never tie an overhand knot otherwise known as a thumb knot), or to put a loop in the rope.
5. Highwaymans hitch has two useful features, first, once tied onto a strong point it has a load bearing end, and a quick tug of the other end releases the rope. Secondly, the knot is tied using a bite so you can tie it and release it without having an end.
With just these 5 knots you can do almost everything.
Each knot is easy to undo even if a lot of force has been put on them.
Used the clove hitch to put two planks together to impressed mutterings from the board staff! Just learn them and they might come in useful on the command tasks. Reasonable amount of ropes involved depending on the tasks set so can't go wrong.