Hi, Festy. I'm pretty new to knot tying myself, and while I think Roo's analogy about English words is a good one, I can resonate with this question. I've learned about 70 knots at this point, so hopefully these choices come from enough of a breadth of options to be a decent selection, but I have a smaller list of 20 that cover pretty much every situation I can imagine encountering (e.g., I'm not a climber, so I'm not likely to need to Prusik my way to safety), so in my opinion you wouldn't need to expand your knot vocabulary too much farther to really have most of your bases covered (knots reliable enough to trust with your life, double and triple loops, more specialized binders, etc.).
I recommend the following, and I'll give my reasons for each. The limitation of your question is that I'm choosing these as the knots a person might find most useful in daily life (the majority of the time). Obviously, if you want to get into sailing or rock climbing, you'll have to get more specialized.
1. Bowline - best all-purpose loop knot, easy to tie; I have to have a particular reason to use a different loop knot
2. Double sheet bend - a good multipurpose way to join to ropes that will accommodate a difference in diameter between the ropes (larger one forms the U, smaller one does the wraps); the double is almost as quick and easy to tie as the single, and will hold reliably in a broader variety of situations
3. Reef knot - my most frequently used binding knot, the easiest and best suited to most day-to-day use; there are better binders for various situations, but very often, the reef knot will do; it's worth knowing the difference between this and the granny knot because the latter is unreliable and can jam
4. Round turn and two half-hitches - works around pretty much any object, and possible to tie under strain, which I find is often the case (you're hoisting something or suspending something or trying to maintain tension in the line when it's tied off)
5. Adjustable grip hitch - a good tension knot (better than the tautline in my opinion) that provides enough tension and grip for most things I ever need to do; I have also used it as a binding knot for cinching tight around a bundle or similar thing; if you have a pick-up and need to tie things down really tight somewhat often, substitute the trucker's hitch
Best wishes, hope this helps.