There have been threads discussing 'best' or 'favourite' knots. The lists were typically varied and somewhat long. So here is a challenge: name only 1 (and 2 and 3) knot(s) that are worth knowing - single knots, no variations (as in e.g. 'bowlines, in general' or both: slipped and unslipped versions as a single knot), no obvious composites (as in e.g. Truckers' Hitch). Complexity/ease of learning or tying/inspectability are not of concern if you only know one knot (or two or three).
For knowing only one knot, I would pick Scott's Simple Lock Bowline (a.k.a. Woven Bowline).
Decent fixed loop; double safety if you can remember only half of the knot (e.g. when times are stressful); can replace many object hitches and some nooses/binding hitches (may not be exactly like Timber/Killick but useful enough for dragging a bunch of Christmas trees behind a truck); interconnected loops form an OK bend (that's not a variation - just tying the same knot twice on two different rope ends); seems to reasonably hold on a variety of rope sizes and materials, including somewhat slippery and/or stiff ones and bungee cords; doesn't shake loose that easily when slack and mostly easy to untie after relatively heavy loading; can be tied one-handed, with the other hand holding the standing part; can even make the Poldo Tackle with two of these...
Also, although this may be controversial given the contest rules, it can replace at least one use of a midline-loop, such as the Butterfly, by using 'the same' knot to attach a second rope in the middle of another one - thinking of the construct as a loop, the second rope masquerading for its working end.
Zeppelin Loop would serve almost just as well, but it requires pre-overhand (I think this is called non-TIB?).
For the set of two or three knots, I think I need a bit more thinking as some choices may work well in combination.