Written Language. Think of how stunted human progress and development would be if we weren't able to keep an objective record of the success and failures of previous generations. Something about re-inventing the wheel. Some of their legacy would last, but not all the knowledge contained therein.
For example: We see the pyramids but don't know exactly how they were built. As advanced as we think we are, we just don't know. If there were a few sentences written and preserved on this subject, there would be no question.
Mass energy distribution is the foundation of almost every advancement in the last 100 years.
(I would put computers at #1 but that also directly resulted in the Internet and Social Media, so it's entirely possible we'd be better off just living in a Steampunk world still using Morse code and Lettermail for communication).