Hello Pradeep

Well, the core branches are those which will remain important forever and have the same value and are thus, evergreen. These basically include Mechanical, Civil and Electrical,because they form the foundation of engineering. The job oppurtunities in these branches are never ending. Comparing them to branches like CS and IT, we see that CS and IT are considered better in the present scenario because they are currently at their developing stage and so have a lot of space and possibility for new innovations and developments. Whereas, for the core branches, most of the things have been developed and there is comparitively less ideas left. But then, the basic point to think about is that, both these types of branches cannot work all alone, they need the support of each other. CSE sector can never develop if it does not have the supply of tools by the mechanical, similarly the old age machines are outdated, thus the CS and IT sector has to help in order to make the machines automatic and fully developed. No computer hardware can work without the intervention of electrical sector. 
Basically, what I mean to say is that it all depends on your interests which field you want to pursue, as all the sectors are equally good and have their own perks.

I hope this helps.