For most metals:

* Compressed air: This is the most common and affordable choice for cutting mild steel, stainless steel, and aluminum. It's readily available and easy to handle. However, it's not ideal for thicker metals or for cutting with high precision.

* Nitrogen: Produces a cleaner cut than air, especially on stainless steel and aluminum. It's also better for cutting thicker materials. However, it's more expensive than air.

* Oxygen: Sometimes used in combination with other gases for cutting thicker materials. It produces a faster cut but can also lead to excessive heat and warping.

For specialized applications:

* Argon: A noble gas used for cutting aluminum and other non-ferrous metals. It produces a very clean and narrow cut.

* Hydrogen: Used in specific applications for cutting thin materials and achieving high precision.

* Mixtures: Specific gas blends are used for cutting various alloys and achieving certain cutting qualities.

Factors to consider when choosing a gas:

* Metal type: Different metals require different gases for optimal cutting results.

* Material thickness: Thicker materials often require higher-pressure gases or gas mixtures.

* Cut quality: The desired cut quality (smoothness, beveling, etc.) will influence the choice of gas.

* Cost: The price of different gases varies, and it's essential to consider the cost-effectiveness of the choice.

* Equipment compatibility: Make sure your plasma cutter is compatible with the chosen gas.

It's always recommended to consult the manufacturer's instructions and specifications for your specific plasma cutter to determine the best gas for your applications.