In M Tech you will choose what you studied in B Tech. and What is best it depends on the criterion of Judging, If you consider interest then it differs from person to person so if some course You may feel interesting but not all. If you go for future job opportunities then almost all the courses have job opportunities but CSE is far better compared to other it offers more opportunities than other. For research, you can go to any one of Your interest. There are some companies which Prefer M Tech students in R &D profile so like many PSUs if you are post graduate then there are good chances then you may end up with Rand D in any PSU.