The toughest exams in India are UPSC Civil Service Examination, IIT-JEE Entrance Examination, NEET Entrance Examination, Graduate Aptitude Test in Engineering (GATE), and Common Entrance Test (CAT). 

Why are these exams considered tough?

1. UPSC Civil Service Entrance Examination: It is one of the most difficult competitive exams in India conducted by UPSC. The aspirants have to cover a huge syllabus for clearing the exam. It consists of three levels; preliminary exam, main exam and interview. The candidates who crack all three rounds are offered posts of IAS, IPS, IFS and so on.
2. IIT-JEE Entrance Examination: It is an engineering entrance examination conducted by Indian Institute of Technology (IIT) for students who wish to gain admission in top engineering institutes in India. The exam consists of two levels; JEE Main and JEE Advanced. The exam is considered tough due to its syllabus as well as high competition. 
3. NEET Entrance Examination: National Eligibility Cum Entrance Test (NEET) conducted by National Testing Agency (NTA) is an entrance examination for candidates aspiring for getting admission for MBBS, BDS and other medical courses. The competition is very high since there are only a limited number of seats for admission across India. 
4. Graduate Aptitude Test in Engineering (GATE): Another tough entrance exam conducted to gain admission into Masters and Doctoral courses in Engineering, Technology, Science and Architecture. The syllabus is pretty huge and many candidates  find it difficult to crack this exam.
5. Common Entrance Test (CAT):  It is an entrance examination to get admission for MBA Programmes in prestigious management and business schools in India. The test tries to evaluate candidates on their ability to handle situations based on Logical reasoning, Mathematics and English.