Thee are certainly plenty of them out there. But what would you consider the most useful, and therefore the ones you need to know cold, that you can tie behind your back, eyes closed, with rain pouring from the skies... Maximum of three, not counting variations.
I would nominate the figure eight, the clove hitch, and the trucker's hitch as three that are highly useful in a variety of circumstances. A few years ago, my number one choice would have been the bowline and its numerous variations. Over the last few years, the climbing community has been moving away from the bowline toward the figure eight because of its demonstrable greater security.
Obviously, the more knots in your reliable repertoire, the better, and there are always knots that are perfect in some specialized applications, but that are not always best in more general usage (like the bow knots we tie every day when we lace up our shoes).
My choices come from a technical climbing and caving background,along with some nautical experience, and that obviously influences my choices. What are your favs, and why?