Which ancient egyptian contribution do you think had the greatest impact on later civilizations? why?

Personally, I would say that it was the Egyptian script, as we actually benefit from this invention very much.
The Egyptians devised a number of scripts, among them the hieroglyphs ( and a simplified, demotic version), and from those the Phoenicians adapted their alphabetic script. From the Phoenicians the Greek adopted their script, and from Green the Romans adopted the script, and we still use the roman script today!