There are some topics which are the most important and scoring topics. Ideally you should try to cover the entire syllabus well. 

Physics :

1. Kinematics – Very simple questions are asked. If you keep the definitions in mind, then all questions will be easy
2. Newton’s laws of motion – Knowing how to draw a free body diagram will greatly help in solving almost all problems of this chapter
3. Simple harmonic motion – Energy conservation method will be useful in finding time period in even the most complicated cases of SHM
4. First law of thermodynamics – Knowing the statement of energy conservation will help you to solve a number of numericals and theoretical questions as well
5. Current Electricity – A good practice of Kirchoff’s law problems will help you to score cent percent in this topic
6. Radioactive decay – The radioactivity decay formula will be the most useful tool in solving almost all the problems

Mathematics :

1. Conic sections – Knowing the formulas well will help you. The parametric form is one of the most useful tool in solving problems of conics
2. Quadratic Equations – knowing the fundamental properties regarding nature of roots / intervals in which roots lie will help you
3. Progression and series – very scoring and simple chapter. Knowing the definitionsand practising ample of problems would be useful
4. Differentiation – Keeping the formulas in mind will help. Geometric interpretation of derivative is an important concept. A number of simple questions are asked which can be done with ease
5. Vector Algebra – Knowing how to visualize the situation will be useful. The concepts of vector and scalar triple products are most scoring

Chemistry :

1. Alkyl Halides – Simple questions based on reactivity / type of reactions are asked and can be cracked easily
2. Electrophilic aromatic substitution reactions of benzene – standard reactions are the most important. Direct questions are asked from this chapter
3. Chemical Equilibrium - Knowing the basic equation of equilibrium along with a proficiency in solving ideal gas equation problems will definitely fetch you full marks
4. Chemical Kinetics – First order reactions are asked in JEE and are easy if you know the basic definitions
5. Ores and Metallurgy - Standard questions are asked which can be done easily
6. Coordination compounds - isomerism is an important but easy topic and you can easily score 100% in this

Thanking You;)