My favorite knot is the evenk knot, it never jams even if its covered in ice. Others faves are the Penberthy prussik, figure 9, figure 8, klemheist, Uni-knot, palomar, lanyard knot for structural stoppers, the perfection loop and the good old shoelace bow.
I was shown a fantastic knot for making loops in light mono-filament by a match fishing ace that has more strength than any other, but I don't know the name and haven't seen it anywhere else. Its a double overhand knot to start with, then the overhand loop is given two full twists. The end loop is pulled through the loop at the end the twists. Anybody else heard of something like it?