This is a tough question. I'd say definitely the spear, man I can't go hungry cause everything without food is pointless.
Domestication of animals(nomads or shepherds), farming, and the boat is probably my second one and the techniques used. Gotta imagine how scary it must of been to not only figure out how to capture the animal but work out what it eats and drinks and provide for it so it survives and gives you a food source.
Farming also provided a second alternative food source working out the rain patterns, the sun cycle and seasons and even the type of land and what sorts of grains can be embedded into the land and how long it takes it to reproduce itself. I honestly think for their time period and the lack of no information exchange(ground zero stuff nothing to cheat from or read or be taught). That's damn pretty genius if you ask me.
Don't get me started on the boat and the exploration activities that it ultimately opened up and the associated tools created to figure out how to get marine life as a THIRD source of food beyond animals and the land was insane.
I would think the third greatest invention was how they worked out to cooperate across multiple un-related individuals in family groups and the systems they employed. No coordination is suicide and nothing is possible cause one person can't do it all and being killed would be highly likely or wiping each other out. I am not sure if I should put this as no 1 cause even food isn't possible if this aint right.
I think Medicine would be the 4th one on my list and I mean the early types and how they figured out what certain plants do or different waters, temperatures, mixing it up, giving it time, conditioning, etc. Nothing worse then being in Pain.
I would have to throw in the fire also damn genius you can't eat with out it, you freeze your ass off at night. My top inventions are from the early humanity and the reason is man you guys dont know what it's like to start with nothing and no REVIEWS. The rest after them had something to look back on and add which isnt hard at all. Plus I look at inventions and see it's impact on me. If u take away all modernity and left me with food, water, boat, and fire and those weapons and techniques from the past, I can cope with it, won't be comfortable but it wont impact me like losing electricity, i still have day as light plus fire, If i lose internet and telephones and cars it would hurt but i can jump on a horse or a camel or boat. The modern inventions wont cripple me thats all i am saying without the early inventions id be dead.