Bowline, with the variation of bowline on a bight. Non slip knot, can be used to raise/lower a person.
Clove hitch, used for tying things up, as in bundles, a tent line to a tree. Variation is clove hitch with quick release.
Square knot, used to join two like ropes together.
Taut line hitch, used on tents as you can release or add tension w/o retying the knot.
Overhand knot. The most simple.
I'd call these the big 5.
I with all of these but would add the round turn with two half hitches. As an Infantry and Special Forces veteran, and a Scout Master, if you learn 4 or 5 of the most useful knots and learn to tie them in the dark when it is raining cats and dogs, then you will be able to get through most things.