Depends what you're doing.
My favourite knot is the bowline. Non-slip loop, very strong, easy to tie, easy to undo. Can be used for tying two ropes together, ropes to things.
Other useful ones (especially for sailing)
* Clove hitch * Reef knot * Rolling hitch * Sheet bend
I overuse the bowline. Never seizes, and it can be tied with one hand around your body (or anything else) very quickly.
I think that method was originally taught to me in scouts as an emergency body lift thing, though it would not actually be ideal for that.
Tying a bowline is one of my fidgets. Also famously has a part in Jaws (1975), though not mentioned by name. ("Hey! I got it!")
I’d add tautline hitch, great to have an adjustable-but-holds-well knot to the repertoire!
The bowline is great because you can tie it one handed.
Knowing common knots is not just useful because they are handy, but they also represent a common toolkit - when sailing with someone, if I see they tie a certain thing with a specific knot, I can do likewise. When I see a knot I recognise, I immediately know how to untie and re-tie it.