It is very hard to tell what is the greatest innovation in the world but for surely it is something related to agriculture and the ability to deliver the best quantity of nutrients and oxygen to the plants. Without such innovations engineers for surely would not be able to design automatic http://www.actechwi.com/technologies-s/1814.htm grow boxes.
I think the innovation that makes innovation possible is the concept of free thought, allowing individuals to freely exchange ideas and pursue dreams for a better standard of living. Without the opportunity to use the human mind for this endeavor by oppression of free thinking would have kept us in the caves.
While for some baffling reason innovation in our culture is benchmarked by the invention of sliced bread, I think the greatest physical innovation may be the wheel. I'd vote for language as the greatest conceptual / abstract innovation.
I tend to side with monoculture farming and the plow as the greatest inventions of all time. Without those we would still be running over hill and dale several hours a day trying to eat instead of being able to sit around and invent ICs.
In my mind the greatest invention is the universal language of mathematics. I would also include the advancements made in agriculture and farming. People tend to explore, invent, create and think more productively when the are not distracted by hunger. (Agriculture and trade was one of the earliest reasons for creating mathematics.)