When a kid in Boy Scouts the knots that I learned - and were useful - were:

Square knot - for joining ropes - though I hear the sheet bend is stronger
Sheet bend - for joining ropes of different thicknesses
Tautline hitch - for tent guy lines with the ability to adjust
Two half hitches - for tying to a post or ring
Clove hitch - for tying to a post

I also learned to use an Improved Cinch knot for tying leaders and lures using monofilament

Since then I've learned the following to be very useful:

Trucker's hitch - for tightening rope tie-downs
Fisherman's knot - for connecting small diameter rope - especially for paracord lanyards (non-breakaway)
Prusik knot - for pulling the ends of tarp out across a ridgeline