Making a good choice of Optional subject is important as you will have to read this subject several times. Following is a suggestion of process in which you can find a good optional subject for yourself.

• Go through all the optional sujects and find which ones are closer to your interest. short list atleast 5 to 6 sujects
• Then go through the syllabus of each of those selected subjects thoroghly so that you will know which one is closer to your interest.
• After this step go through the previous year question papers  and guage youself with the levvel of interst and knowledge of that subject.

Then the common factors come in which are equally important like --

• Success rate of the subject you have chosen,
• Avalibility of Notes and guidance 
• Amount of syllabus clashing with UPSC mains GS paper syllabus - This will make you save time in your studies for optional subject.
• Amount of static syllabus- Subjects like History, Geography  , languageshave less amount of Current affairs. So it Requires less amount of updation, in comparson to the subjects like science subjecs or economics. This does nt mean that the subject with current affairs are bad, or less scoring, Many toppers had chosen subjects with more Current affairs. It all depends on your comfort level with the subject.

By this time you will be very clear about the subject you would like to take as your Optinal for your IAS Mains Exam.