The most useful knot for #SOTA and #POTA in my book: Ian's secure knot. Originally a shoelace knot (I never had this undone while hiking, but it is easy to undo intentionally). It is very useful when tying a mast to a fence, tree, etc., because the knot is secure against the motion/vibration of the mast. Most other knots/hitches (like constrictor) are weak when the mast moves inside the knot. Highly recommended, especially for Spiderbeam 12m HD!
https://www.fieggen.com/shoelace/secureknot.htm
See Parts 2 & 3 for more
Part 2:
https://www.animatedknots.com/double-dragon-loop
More secure but not much more work than bowline, easy to remember and undo. A loop can be made in the middle of a rope without access to either end, so it can replace Alpine butterfly.
https://www.animatedknots.com/buntline-hitch-knot
Versatile. Classic. It's actually quite fast with the finger wrap technique. Use slip knot to facilitate undoing.
https://www.animatedknots.com/farrimond-friction-hitch-knot
A super convenient friction hitch when the rope is very long (no need to access the end of the rope). Easy to undo.
Part 3:
https://www.animatedknots.com/constrictor-knot-rope-end-method
Double constrictor is an even more secure variant. Very useful when tying things down to vehicle/trailer, tying wound cables, etc. Best in slip form for undoing.
https://www.youtube.com/watch?v=JaLyC-h09k0
Woodland zip tie is useful when carrying a jacket tied to the pack, etc.
I use NiteIze CamJam to replace trucker's hitch. It's significantly less work and places where leveraged tension is needed is uaually known in advance.
CamJam is also good for adjustable guy wires.