Hello,

In order to appear for UPSC, candidates need to pass graduation in any discipline from a recognized University, there is no specific aggregate required in this case, you need to attain 21 years age, and can apply even in the final years.

There isn't anything as such best subject for UPSC, as it's tend to be more humanities oriented, hence those from the similar backgrounds have added advantages of being well acquainted with the subjects which don't happen in case of other students such as engineering or medical, however if we look at previous year trends of toppers, many people from these background too have successfully qualified, so eventually it's your planning, preparation and dedication that matters, also UPSC tends to be unpredictable, in such cases it's good to have back up plan.

If you are very clear about appearing for only UPSC, and no other exams, you can pursue graduation in any discipline of arts such as  political science, public administration, economics, geography etc, however in this case you have start your preparation from college year itself as upsc syllabus is humongous.