There are different approachs to this question, but in my opinion I think that natural gas is not the most harmful fossil fuel to the environment.
Since natural gas is Methane (CH₄) it burns cleaner than other fussil fuels like coal for example.
Natural Gas 0.2 kg CO₂/kWh
Gasoline 0.25 kg CO₂/kWh
Fuel Oil 0.28 kg CO₂/kWh
There we have 3 fossil fuels. If we look at them we will see that if we burn Natural Gas we can produce more energy with less emissions of Carbon Dioxide.
