M.S

Dear Sravya,
Always understand the term best will depend from person to person. Mechanical, automotive, computer science, business management, etc. are some of the top choices for students. USA is a vast country, job scene differs from coast to coast. It is best if you know your career plans, where you want to settle and check relevant information.

Cost of doing B.Tech and MS from USA is very high and Return on Investment (ROI) will be very low and poor. With USA work visa regime becoming tougher, you may not able to work in USA and earn in dollars to repay any loan. Therefore, I will like to advise you to do B.Tech from a good engineering college in India and then MS in USA.

If you have already done post graduation (M.Tech) in Aeronautical Engineering then you need not do MS as both MS and M.Tech are similar degrees. If you have done only B.Tech Aeronautical Engineering then MS or M.Tech will definitely be a value added additional qualification.

Yes, civil engineering is a demanding profession in Australia. Fee will depend on the university. It can range from AU$20,000 to AU$40,000 a year. You can check details on university websites.