I have an Opinel pocket knife which, when I lose it, causes me to feel bereft. I even have a spare one, because I lost it for six months or so, but once refound the spare was put away for emergencies.
However, the most useful tool on the farm is a 4 foot iron bar with a sharp pointy end and a flat tamping end. If you can't move it with the bar, you can make it smaller until you can move it with the bar. You can dig holes with it, you can uproot trees with it, you can scrap and disassemble cars with it, and you can frighten the neighbours with it if necessary. A vital piece of kit.