What gas is the most important?

Answer 1

This depends on what you mean by important. For instance chlorinated fluorocarbons at a given concentration have an effect thousands of times that of the same concentration of Carbon dioxide. Fortunately their concentration is very low. On the other hand water vapor has a lower effect than an equivalent amount of carbon dioxide but its concentration is very high (not surprising since two thirds of the earth's surface is water) so with the current composition of the earth's atmosphere water vaporhas the greatest effect. However the concentration of water vapor varies with the temperature and is not directly affected by human activities. If we were to increase the level of water vapor in the atmosphere and leave everything else unchanged, the water vapor would fairly quickly condense out as rain, snow, frost or dew and there would be no lasting effect on global temperatures

Carbon dioxide comes second after water vapor and its concentration in the atmosphere is heavily affected by burning of fossil fuels. As the CO2 concentration increases, the temperature of the atmosphere increases, as does its ability to hold water vapor. So an increase in CO2 results in an increase in water vapor and a further increase in temperature. Because CO2 does not condense out it has a lasting effect which is amplified by the ensuing increase in water vapor.