The greatest use of water is a regional answer. However, overall energy creation is the greatest use. Coal, gas, radiation are all used to heat up water. Hot water creates steam. Steam moves turbines to create electromagnetic energy and we turn on the lights.
The answer to your questions implies a judgement on "inappropriate" use.
70% of the worlds water use is for agriculture. My reply would therefore ask a refined question as to what the most inefficient use of water is for agriculture that with known technologies could be remedied.