In one line I would say Mechanical, Why see almost every manufacturer needs some kind of Mechanical Equipment. It is a core branch of Engineering which has many sub-branches such as automobile, Aviation, Tools. Everywhere You can Find Mechanical engineers. While aerospace is very specific and has limited scope and narrow field so In general you will be offered fewer opportunities compared to Mechanical, one more thing is that You can go to Aviation sector even after Mechanical Engineering but not vice versa. 

If you consider Job scenario then almost every Private or Govt Enterprise need Mechanical Engineers, even you can choose this as a subject for civil services main but not the case with other Branch.