The Bowline is definitely the only knot to learn if you have to choose just one. There are 3 (that I know of) different ways to tie it that are worth learning

The Bowline is definitely the only knot to learn if you have to choose just one. There are 3 (that I know of) different ways to tie it that are worth learning as a next step. One of them lets you prepare it so you can quickly tie it around something and have it invert on itself to form. Another technique actually revels that the bowline is the same as a sheet bend, just using one line folded back on itself to make a loop instead of 2 lines joined. So that’s a freebie (and the sheet bend is a great knot). It’s super versatile. I never use the reef knot anymore. Lost a dingy in Greece because I trusted it for about 5 minutes. I find it works itself free really easily.