That said, the most commonly opted-for subjects for the optional paper are Geography, Public Administration, psychology, political science, History, Sociology, Philosophy, anthropology, etc.
However, according to me, you should always choose the subject that you are most comfortable in.
There are a lot of optional subjects to choose from when it comes to the UPSC Mains. There are 25 subjects in total. To know the names of the optional subjects, you can read the blog that I have written on the topic of the UPSC IAS syllabus.