It would be the water vapor. Because while ozone gas reflects xrays and ultraviolet radiation, they do not block infrared radiation. It is the infrared radiation that we know as heat, which is the cause of Global Warming.
Why water vapor - it is all about the albedo effect. Clouds are mostly consists of water vapor and as we all knows, clouds are white. This is why clouds reflect more heat into space than anything on Earth has. Then the second best reflector infrared heat on Earth is ice. Ice is still made of water and is white. That is why many have feared that the more heat there is on earth, the less ice there is, which is bad because there would be an acceleration of heating on Earth.
Finally there is one gas out there that reigns supreme, and arguably better than water vapor - Sulfur dioxide. However SO2 is a temporary greenhouse gas which usually settles down a few years after it was sent into the upper atmosphere. Sulfur dioxide, when in contact with water vapor becomes sulfuric acid, however, because of the cold temperatures in the atmosphere, they became sulfur crystals. However, this gas is bad because it causes acidic rain down below, which is sometimes deadly for earth's plantlife and us as well. This gases are only present in the atmosphere after a large volcanic eruption, and the largest one was Mt. Pinatubo, which cooled down the planet by an average of 2 degrees. This gas is so effective, it was argued that SO2 helped on killing the dinosaurs by cooling down the Earth to almost freezing temp. Then there was also the Toba eruption which sends a lot of SO2 in the atmosphere 75000 yrs ago that almost pushed us to the brink of extinction.