Hello,

It's quite difficult to label one optional subject as best option regarding UPSC, the reason being if we go through previous year toppers they have diverse options such as the one with engineering subjects have qualified the exams, one of the toppers from IIT background had physics as optional, and then of course we have subjects like anthropology, geography, political science, public administration, law, sociology to name a few, now you have to consider few things prior to deciding upon your optionals such as your interest, that's extremely important, more so if you haven't studied the subject at graduation level as you have to start afresh, look out for availability of study materials in your chosen subjects, and you can also go through toppers interview regarding this, for instance you can get to know about detailed information regarding the preparation portion, see the syllabus of optional subjects in upsc is equivalent to graduation level studues, hence in majority cases either people go for the subject they have done hons in or at least they have studies the subject at graduation level and have good grasp regarding the same, like I mentioned about previous year engineering graduate from IIT who went ahead with physics optional.