Human Society..... but seriously Weapons. Without them we would be wiped out by predators and not be able to hunt for food. Needed to kill the mammoths for food with a spear you know not to mention it's fur would be good for warmth. Can't build a fire everywhere or in the rain. Also the wheel is for transportation, back then hunting herds of animals was the most important thing and the wheel can't go everywhere.
Taking craps on the street hardly matter if there are only few thousands of us scattered all over the world. That is why weapons are our greatest inventions because it keeps us ahead of animals and each other. My tribe back then is not your tribe's friend was the mentality.
If we are talking modern age inventions then Indoor plumbing if only to keep disease to a minimum but that is only in 1st and 2nd world countries. 3rd world countries don't even have that. Farming would be 3rd world countries greatest invention to feed themselves.
Although it might shock some of you the Internet is hardly Man's greatest invention. All it does is allow better communication and it branched off to include entertainment value.
This is just my opinion though so if you think the Internet is the greatest invention we humans have then w/e. This is all subjective opinions anyways.