Kinda depends on the engine. You want a thick washer that has the hole the exact size of the prop shaft, and has the hole exactly centered. Both primarily for keeping out-of-balance vibration from happening. For most, a good quality nut of any kind will work - something you can tighten down well. Some engines, primarily larger four strokes, had a special double compression nut arrangement, to better reduce the chance (especially if it kicked back) of the nut/washer/prop from spinning off the shaft. Some had nuts that fit into a recess in the washer. Original equipment is the best, but I'd guess that many have substituted hardware store items, although the risk may be a bit higher. Extremely rare to have a prop come loose in normal running. It almost always happens during startup due to the electric starter, kickback, or fuel lock when flooded.