Good Afternoon

Hope you are absolutely fine, I think you mean best optional subject for UPSC.

firstly it is depend on you which one is easy subject for you and interesting one, so here is the best optional subjects for UPSC: Public Administration, Sociology , Philosophy, Political Science & International Relations ,Geography, Anthropology, Chemistry, Accounts and Agriculture. these courses are considered as the best optional subjects for UPSC

For more information about UPSC checkout given link below:
https://competition.careers360.com/exams/upsc-cse

Hope you found it helpful
All the best!!