"Mutual, unconditional love". Not sure it is universally accepted as No.1. The Buddhists speak of "loving kindness" and the closely allied "compassion", neither of which is a perfect translation of the original Pali terms, apparently. But in their eyes, requiring the feeling to be mutual is a form of attachment, which is the source of unhappiness (also not a perfect translation of the original Pali, apparently.) So maybe just "unconditional love"?
Heavy stuff, maaan. As Neil from the Young Ones would say. More manageable maybe, mankind's greatest invention. The one thing that made the wheel and IC engine possible? Maybe fire. But that was a harnessing of a natural phenomenon rather than an invention. From a utilitarian point of view, the greatest invention would be the one that has done the greatest good. What would that be? Modern drugs such as antibiotics and vaccinations that have saved countless millions of lives? The Green Revolution of high-yield, disease and pest resistant crop strains combined with modern fertilizers that saved millions from starvation in Asia in the 1960s-70s?
Or was it the animal-drawn plough that allowed the agricultural revolution of 6,000 years ago that enabled the civilization of the Northern Hemisphere? Gave the city dwellers the wealth and leisure time to pursue arts, philosophy, science, education, medicine and the like, including democracy. One theory is that today's high poverty level in most of the Southern Hemisphere goes back to the lack of any kind of native beast of burden to haul ploughs or carts. So the invention that made the difference between the life of the average person in Europe today and the life of the average person in most of Africa, for example, is certainly pretty great.
ISTR at the turn of the millenium, Caxton's printing press was lauded by the media as the greatest invention of the past millenium, bringing the spread of knowledge and education through books at a scale hitherto unseen. But the media may have had its own biases about the importance of the dissemination of information, clouded by the big enthusiasms everyone had for the internet at that time.