Question
Dear Ronald

Is Urea the most preferred fertilizer for
farming?.

Thanks
Prashant

Answer
Prashant, you have asked a loaded (very loaded)question! You are either testing my knowledge; in which case, forget it!
Or you do have a genuine difficulty and you think that I can help. If it is the latter scenario, I feel quite honored that you have done so; and will do my level best to help - BUT ! It might take some time.
There are three numbers which must - by law, in the USA - be displayed on the container of fertilizer which you might purchase. The first number tells how much Nitrogen is in the product.
Nitrogen is one of three elements which living plants need - in large quantities! The major elements.
Why? Because nitrogen gives plants that green color; and that, in turn, helps a plant to manufacture its own food - directly - or our food - indirectly. Nitrogen makes living plants green and stimulates them to grow in size! Remember that. Complexity is another parameter of growth.
The number itself is a direct indication of how much potential nitrogen(%N) is in that bag or box.
Making it available to the plant is quite another matter.
So far, I have been writing about what nitrogen means to us, humans.
Let us consider what nitrogen means to the plant. Nitrogen is nothing more or less than an element that the plant uses to nourish itself, directly; or us - undernourished humans - indirectly.
If we humans were not there to provide the fertilizer, how would the poor plant obtain the all important nitrogen for its livelihood?
We must ask Mother Nature.
Isn't Mother Nature marvelous? She provides! If only we make the effort to seek. Mother Nature invented the "Nitrogen Cycle" - and they taught us about it in high school. The nitrogen cycle is the process by which nitrogen, an element in the air which we breathe, is converted into a molecule of nitrate which plants readily use.
There are many sources of nitrogen which we can offer to the plant and leave it there; for the plant to take or leave as it chooses.  
Historically, traditionally, we applied (pen, farmyard, cow, horse)-'manure' to our plants and crops; and obtained increased yields.
The farmer harvested more corn, wheat and potatoes; and his wife picked up more eggs and obtained more milk - for the same effort. We, many generations later, still reap some of the benefits from those laborious efforts. We now know, thanks to a large group of scientists, that these organic (caution!!! with this term) products provided the environment and / or the raw material for minute organisms to live and thrive in the soil.
The topsoil is a living, pulsating environment, exemplified by the 'floor' of the tropical rain forest. There, plants and animals live and die in an ever changing, yet sustaining, sort of "web" which would perpetuate itself indefinitely; if we humans, did not (have to?) intervene.   
For many and very good reasons we have had to increase the yield per acre for the major crops - wheat, rice and corn - in order to feed a growing world population. The development of new varieties and the improved nutrition of all crops have been the two paths to this objective.
Many sources can provide nitrogen to a crop.
 Urea is the most concentrated - 46% nitrogen. It is very soluble (and volatile!); consequently quickly leached away. Moisture is critical to its uptake by plants; or else serious crop injury can result. It requires massive amounts of electrical energy for its production; therefore quite expensive. Its use is justified economically in the production of high priced crops and intensive crop production; eg hydroponics.  
There! I hope that I have provided an answer that is incomplete enough to stimulate your imagination to seek further.
I, myself enjoy this kind of conversation and hope that you also do.