Written language, may be the most important but lets face it due to this little discovery we also have to deal with things like Paul Ruebens Auto-biography.
I knew someone would mention this. True, fire as a natural phenomenom was a discovery. However, the ability to create fire AT WILL and use it as a tool is, in my opinion, an invention.
As much as I love technology, I think social inventions have been the most important. Laws, property rights, democracy. These are harder to pin down to one individual or time.