The soil water that is of the greatest importance to plant life is capillary water. Capillary water refers to the water that is held in the soil against the force of gravity. It is the water that can be absorbed by plant roots and used for plant growth.

Gravitational water, option 1, is the excess water that drains through the soil due to gravity and is not readily available to plants. While it is important for maintaining soil moisture levels, it is not directly utilized by plants.

Hygroscopic water, option 3, is the water that remains bound to soil particles and is not available for plant use. It is tightly held by soil particles and cannot be absorbed by roots.

Combined water, option 4, refers to the water that is chemically bound to soil minerals and is also not directly available to plants.

Therefore, capillary water, option 2, is the correct answer as it is the water that plants can absorb and use for their growth.