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