Hey Arshan!

You have not mentioned the subject for which you want to know the most important chapter from the exam's point of view. In your board exams, each subject is equally important as all the subjects have equal marks weightage in the exam. Moreover, you will have to try to cover each chapter of each subject while doing preparation. Since you haven't mentioned the name of the subject, I would suggest to prepare the main and most important concepts of each chapter. You must focus on those concepts which are repeated each year in the exams. However, for excelling in your exams with good grades and percentage each and every chapter in the syllabus and all the subjects are necessary.

All the best!