I am not sure whether anyone will have a correct reply to this. If there was, all mechanical engineers would be doing it. Anyway, primarily earning money never depends upon the branch. It will solely depend upon you and your capabilities. However, you could earn well by normal standards and have a secure job in PSUs. For that there is normal recruitment through the job channels and then there is GATE. Qualify in it and you could get a job at one of the leading PSUs - ONGC, BPCL, HPCL etc and they pay very well. 

Mechanical engineers can have careers in the merchant navy, maintenance - jobs in gulf are mostly for plant maintenance and stuff; you could check the banking recruitment - they also take in engineers; there is the Indian Engineering services which is conducted by UPSC and you could be getting a job in the engineering services of the Govt; there are the defence jobs.

If you are not one for jobs, you could look at a start up and then build your own venture. 

Now you have to sit and figure out what suits you best and get going. All the best!