A lot of people consider the IAS exam to be the toughest exam in India. The only reason behind this is because of the meager pass percentage here. Also, there is a huge syllabus to be covered not to mention the variety of topics one has to learn for it. And, it is not only knowledge but your writing skills also that are tested. Once you get through the written parts (prelims and mains), you have to face the IAS interview/personality test, where you will be assessed on your overall personality, administrative skills, and ability to face pressure. A lot of this makes the IAS exam seem really difficult to crack.