This is the most challenging question. "Most important" is always answered from the perspective of values.

☛ To many farmers, the most important thing about soil is that it produces crops that provide their livelihood.

☛ To environmental engineers, the most important thing about soil is its ability to purify itself.

☛ To engineers who build things, the most important thing about soil is its ability to support weight or loads. (The leaning Tower of Pisa leans because it was built on soil that could not support its weight.)

☛ To people concerned about wildlife, the most important thing about soil is its role in providing habitat (food and shelter) for many wild animals and several endangered species.