There are too many knots to mention!
However, from personal experience, I have not had the standard Improved
clinch knot fail on me.
This serves my criteria of having one knot that I can tie well, even if my hands are cold or the light condition is poor or it is windy. I can almost tie it blindfolded.
This knot is used for fishing moderate size fish - anything larger
(GTs etc. on poppers or trolling ) seems to be the domain of the loop knot.
Leader length, usually about 3-4 feet. In case of heavier fishing, the leader may well have to be double or treble that length, as the doubled mainline/leader connection may be too big to pass through the rod tip guide and you will need the extra length when trying to land the fish at the boatside.
You may also need a longer leader when finesse fishing ,for say trout,
as these can be spooked by coloured line and a flurocarbon leader will
reduce the risk. Because a thinner line/leader is used in this instance, the
joining knot will travel through the rod tip guide easily.
Tight lines (literally)!!