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.
This is almost verbatim what I would have said. Instead of the tautline I would have said 2-and-a-half hitch, but they're pretty similar (if I remember my knots correctly)
Don't forget the regular shoelace knot - good for what it does only, but you use it literally everyday. And I like a hangman's knot, just for kicks.