-- the bowline, for putting a slip-free loop around something -- two half hitches, for putting a cinching loop around something -- the clove hitch, for tying a rope to a pole or other cylindrical object
Start with those three, tied properly, and you can do a lot.
I also 2nd the truckers' hitch.
And remember: if you can't tie knots, tie lots
Bowline -- outrageously strong, simple to tie, easy to undo, can tie it around things.
What can be better?
Bowline requires stopper knots when used on on modern ropes, otherwise it can slip. I've never seen kalmyk loop slip.
Both are simple and can be tied around things.
Kalmyk loop (which is exploding variant of eskimo bowline) is probably not secure enough for rock climbing, but better than bowline for other tasks. Less likely to spill and easier to untie.