The bowline knot is the classic climbing knot and it indeed is highly useful, but even better, and now more common in use by climbers, is the figure 8 and a closely allied knot, the "figure 9."

The clove hitch is an extraordinarily useful and versatile knot. Very easy to learn and apply.

The square knot is way overrated. It is best employed to tie shoelaces and nothing more critical. The fisherman's knot (or triple fishermans) is much more secure for joining two ropes; the water knot is good for joining webbing.

For climbers, the prussik (or several similar) is well worth knowing. It is very closely related to the taut line hitch

One reason there are so many knots is that different activities require different means of joining and manipulating rope. There are considerable differences between what is employed in climbing compared with boating. So, what specific use are you considering? That determines the knots you should learn.

By learning, I mean the ability to tie the knot in a dark shower with your eyes closed and the cold water running full. It is better to learn a few knots well than a whole bunch half way.

If you are tying knots for any serious purpose, they should be backed up - typically with two half hitches.