gautam.sanskriti@gmail.com, 12th Standard asks: Should we choose universities based on their overall rankings or on the basis of our major rankings?

A combination of both would be ideal! Many universities, although ranking extremely highly, do not particularly stand out or excel in every academic field, and one may doubt the quality of education there. However, the relative weakness of that department is usually made up for by the unparalleled amount of resources and networks such universities can offer, along with the instant brand recognition that comes along with it. As an example, the computer science and engineering school at Vanderbilt does not rank as well as compared to other US News Top 20 colleges, but the name and prestige of Vanderbilt usually offsets the disadvantage.
On the other hand, many universities like UIUC are known especially for their strength in a particular department (in this case, computer science) - these can also be a great option if you are absolutely sure about the industry you want to go into, as they are well-recognized by experts in your potential field.

Hi!
I think the previous answer was quite comprehensive but another thing I wanted to add is that you shouldn't just go by rankings when you decide where to go to college, because it's quite likely you'll change your mind a lot as you go through the process. Also consider quality of life factors like location, weather etc as well as other academic factors like the presence or absence of a core curriculum, minors etc.