Final answer:
The most important factor in George Hadley's single-cell circulation model is the unequal heating of the Earth's surface, primarily due to the direct sunlight at the equator and oblique rays at the poles.
Step-by-step explanation:
According to the single-cell circulation model proposed by George Hadley, the most important factor causing the global atmospheric circulation was the unequal heating of the Earth's surface. The planet receives more heat at the equator due to the Sun's rays hitting the surface more directly, compared to the poles where the rays are more oblique. This difference in heating causes air to warm and rise at the equator, creating a low-pressure area, and then cool and descend at the poles, generating a high-pressure area. This circulation is further influenced by the Coriolis effect, due to Earth's rotation, which causes the deflection of air currents, creating the easterly and westerly winds.