I'd 2nd the bowline - very useful knot. I was once forced to learn to tie a bowline behind my back - seemed fairly useless at the time, but then a few months ago I had to tie something onto the back of the wetsuit that I was wearing - to my amazement I did it first time - muscle memory is an amazing thing.
I also 2nd the truckers' hitch.
And remember: if you can't tie knots, tie lots