Currently, the most common refrigerants used are hydrofluorocarbons (HFCs), such as R-410A, which have a lower impact on the ozone layer but still contribute to global warming. However, HFCs are also being phased out in some regions due to their high global warming potential.
The most environmentally friendly refrigerants are natural refrigerants, such as ammonia, carbon dioxide (CO2), and hydrocarbons (such as propane and butane). These refrigerants have zero or low global warming potential and are non-toxic, but they may require special safety precautions due to their flammability or toxicity.
Overall, there is no single "best" refrigerant gas that fits all situations, and the choice of refrigerant will depend on various factors, including the specific application, safety requirements, and environmental considerations.