Which stream should I opt to become an aeronautical engineer: Mechanical engineering or Aeronautical engineering?

Aeronautical Engg is obvious choice.

For becoming aeronautical engg. opt for B.Tech/ BE in aero, but if you choose for B.Tech in ME or mechatronics one can also get job in aeronautics. The best colleges in which you can look for admission are IIT Mumbai, IIT Madras, PEC Chandigarh, Lovely Professional University.