You could argue that water vapor is actually the worst based on it accounting for anywhere from 33% to 85% of the greenhouse effect. It’s also believed that water vapor concentrations will increase with the increase of other greenhouse gases, thus exacerbating the problem.
Methane (CH4), nitrous oxide, HFC’s, and sulfur hexafluoride (and several others) are all worse in the sense that they all have greater global warming potential and may be more difficult to remove than CO2. Additionally while CO2 is by far in greater production than any of these, we understand and have the means to significantly reduce its production today where the other gases would take more research, money, time, and consideration before being able to be as effectively reduced; thus implying that CO2 is no longer the threat we should be focused on.
You could even go as far as to explicitly blame CH4 over CO2 because if you take combustion as the primary source of CO2 the particulate matter the process expels actually has a countering cooling effect. Thus so long as we are consistently increasing the particulate matter in the atmosphere the effects of CO2 are mitigated.
Further CH4 is believed to have a pretty dramatic feedback effect which makes it even more dangerous, particularly at this point where the question of CH4 vs CO2 concentrations escaping from permafrost could point to CH4 making a much more dramatic rise than previously expected. although now we’re talking about future effects
Keep in mind the focus of all of these points is only to back a claim that CO2 isn’t the main cause of global warming (it could even be more narrow, saying “today it is no longer the biggest threat”). Very effective counter arguments abound, the best you can hope for in such an argument is to get your opponent to agree that there are other things just as potentially dangerous. Instead of focusing on bringing down only CO2 emissions we should be focusing on a broader range and getting others such as CH4 under control too. If you try to use this in a debate to claim CO2 isn’t an issue and we need to abandon our efforts to curb it, frankly, you’re screwed.
If you want the details to support this you’ll have to dig them up (or fork over some class credits) wikipedia should have most or all of it.