Over time, many automobile air conditioning systems lose their refrigerant charges. This is due to leaks in these systems, and the best practice is to find and fix the leak before adding more car refrigerant. However, due to the costs involved, many choose to ignore small leaks and simply top up their systems every so often. This is one of the reasons that Freon was banned; it was continually being leaked into the environment.
                         
Now, the best car refrigerant offerings do not have such harmful environmental effects. While it's still better to find and fix leaks, topping-up isn't as bad of a practice thanks to this change.
                         

The best car refrigerant offerings make it easy to add refrigerant to a system. They come in dispenser cans that attach to the charging port and release their contents when triggered. In most cases, there is no need for hoses or other equipment. However, if a charging port is extremely hard to reach, it's a simple matter to attach a special extension hose. With this type of hose, you still gain the convenience of being able to just trigger the refrigerant can and have the rest take care of the rest, with the exception of needing to attach and detach the hose at the proper times.
                         
Importantly, the main thing that makes a car refrigerant best for your vehicle is that it matches the manufacturer's specifications. Do not substitute one type for another even if it is rated highly. Using the wrong one can destroy your air conditioner.
                         
Other than that, look for cans that are free of rust and that appear to have valves that are well-made. This will ensure that it is uncontaminated and easy to install.