1. Square Knot / Reef Knot -- This is the shoelace tie, and its one of the simplest knots. Learn this knot backwards and forwards you'll use it a lot. Avoid the granny knot (a mistaken reef knot). Beginners will often tie the granny knot instead of the square knot. If you can do a square knot, the "Surgeon Knot" is a useful variant which is more secure.
2. Figure Eight -- Superior to the overhand knot in almost every way imaginable. Never use an overhand knot, always use figure eight instead.
3. Bowline Loop / Sheet Bend -- When a loop must be sturdy and unmoving, the bowline is one of the best. The Sheet Bend is nearly the same, except you tie two different ropes together. I keep a bowline loop on any "pocket twine" because making a bowline loop is the start of many advanced knots. Such as...
4. Package Tie -- Presents obviously look nicer with a ribbon in package tie form, but any large, box-ish object can be secured with the package tie. Really good for lunchboxes / bags in daily usage. Add another Bowline loop on the end for a convenient handle. https://www.animatedknots.com/package-tying-knot
5. Cable Lacing -- When working with wires and electronics, the cable-lacing knots are great for organizing wires. Its quick to tie and organize, and looks great. https://www.flight-mechanic.com/lacing-tying-and-cutting-wir...
6. Poacher's Noose -- When a loop needs to "collapse" to bind objects together, various nooses come in handy. Poacher's noose is my favorite, ties easily and fewer people recognize it compared to the hangman's noose. Both nooses have political implications however so be wary of who is watching you when you tie a noose.
7. Butterfly coil -- When storing most paracord, twine, or ropes... butterfly coil is fast and effective storage.
8. Over-under coil -- Wrapping up thicker wires / cables (especially electronic cables / wires, but also garden hoses) is best with over-under coil instead of butterfly coil.
9. Half-hitch -- Its the knot you've been using all the time, and you probably don't even realize it. Its called a "half-hitch", because you need two of them to actually make it secure. Two-half hitches solves many problems.