If it's a safety critical situation, leave it to the experts/become an expert/try to make it so that the failure of your knot won't kill anyone/etc.
For non-critical use(In my limited experience, some of my choices are as much about ease of tying as actually picking the right one, some of these are things I've only used once or twice or never used for anything important):
If you want to tie two ropes together, use a sheet bend. or a double for more security, or the slipped version to be able to undo it. Add a stoper knot for even more security.
If you want to make a loop in the middle of a rope, alpine butterfly.
If you want to adjust tension, tautline hitch.
If you are tying thin spectra cord, or making a permanent attachment between thin cord and something else, try a uni-knot. Note that it's not actually considered the standard for tying to carabiners, but I use it sometimes(For non climbing purposes).
If you are tying a bundle together, try gliepnir or a standard shoelace knot, or a clove hitch.
If you are tying a rope to a fixed pole, try a clove hitch. It's not perfectly secure. Maybe try gliepnir too. Honestly bundles and hitches are the two uses I'm least sure of. There are apparently better things than the clove hitch, but it's easy and good enough probably.
If you want to tie something you can undo really, really fast, use a highwayman's hitch.
If you want to tie something someone else might need to untie, shoelace knot which is a doubly slipped square knot, it advertises visually that it's meant to untie.
Might want to just use a regular overhand though, the Ashley stopper is hard to undo.
If you want to cause a real nightmare for anyone untying something, or remove unwanted fingers by circulation loss, constrictor knot.
If you want to make the end of a rope not go through a hole, Ashley stopper. You can also use this, or just a simple overhand, to keep another knot from slipping.
If you want to do something involving climbing, do not listen to me, I know nothing about it.