What contribution is regarded as the most important piece of 'intellectual technology' he left behind?

His iterative, human-centered design process methodology

While the mechanical designs often receive the most attention, a deeper analytical perspective suggests that Da Vinci’s most profound contribution might be methodological rather than artifactual. His consistent approach—which involved meticulously observing nature, sketching potential solutions, testing those sketches theoretically through calculation or comparison with natural models, and subsequently iterating on the design—is recognized as an early, highly effective form of design thinking. This structured, human-centered process for approaching the unknown is considered by some analysts to be the single most important piece of 'intellectual technology' he bequeathed, forming the modern cornerstone of innovation centuries before it was formally codified.