The following are the latest estimates of annual global emission for nitrous oxide, methane and carbon dioxide (in gigatonne):

N2O: 0.007 Gt per year

CH4: 0.56 Gt per year

CO2: 35.9 Gt per year

A picture is presented in the link:

https://www.epa.gov/ghgemissions/overview-greenhouse-gases

The question has several undertows and therefore not straightforwad in answer.

Firstly if we are to consider the upper atmosphere then these global warming agents lose their moniker as the high concentration of GHG is actually making the upper atmosphere cooler; the warming effect is only felt in the lower atmosphere.

AND, if the gaseous phase is to be considered solely then nothing serves more as a warming agent than water vapour. Otherwise it is as Joshua O. Ighalo , Y. Jun Xu and Roberto Molteni have already pointed out.

The relative green house efficiency compared to CO2 as one,

CFC-15000, N2O-230 and CH4-25.

But green house contributions are,.

CO2--57%

CFC-25%

CH4-12%

N2O-6%

There are several gases responsible for global warming.

As per as agriculture sector is concerned, nitrous oxide is more important followed by Methane then CO2, it terms of Global warming potential.

But all are expressed in CO2 equivalent

bulk wise CO2 is important

This is not about emissions alone

It is emission x absorption of outgoing terrestrial radiation x residence time in the atmosphere

This is a simplified summary of what is written in the Encyclopedia of Climate Change:

The "IPCC-report" and I do not understand that people here present their opinion instead of referring to such an authoritative textbook?!

" Global Warming Potentials and Other Metrics for Comparing Different Emissions"

https://www.ipcc.ch/site/assets/uploads/2018/02/ar4-wg1-chapter2-1.pdf

It is well documented that GHG emissions from agricultural land is rising due to anthropocentric activities such as burning fossil fuels and land use change. This has resulted in increasing a global atmospheric temperature up to 0.8° C during the 20th century, with likely increasing to 1.4°C –5.8°C in the 21st century. Although, CO2 is the major reason of increasing global temperature, the global warming potential (GWP) of CH4 and N2O is 298 and 25 times greater than CO2, so the formers can trap more heat in the atmosphere compared to the latter

You can see both next ref.

Ali, M. A., Kim, P.J. and Inubushi, K. (2015). Mitigating yield-scaled greenhouse gas emissions through combined application of soil amendments: A comparative study between temperate and subtropical rice paddy soils. Science of the Total Environment 529: 140–148.

Mandal, S., Sarkar, B., Bolan, N., Novak, J., Ok, Singh, Y. S., L. V. Z. B. P., Kirkham, M. B., Choppala, G., Spokas, K. and Naidu, R. (2016): Designing advanced biochar products for maximizing greenhouse gas mitigation potential, Critical Reviews in Environmental Science and Technology, DOI: 10.1080/10643389.2016.1239975

CO2, CH4, NOx and CFC

Yes cow is most contributing Gas for GH effect

It is NO2.

I agree with Rocío Millán

Joshua is right

I think burning fossil fuels for electricity, heat, and transportation

It is NO2