hi rahul, i can tell you based on previous year split up, but mind it, it could vary next year,

Physics: Mechanics is the most important topic. 12-13 questions were asked on it last time.

Chemistry: Organic Chemistry dominates the chemistry question paper. 14-15 questions were from topics related to it.

Biology: There was a more uniform distribution of questions in Biology. There was no single unit or topic from which more than 6 questions were asked. Hence it would be wrong to deem any one single chapter most important as all are important

While the NEET question paper has variations every year, the topics that contained the highest weightage in NEET 2017 are as listed below:

Physics: Thermodynamics and Kinetic Theory, Mechanics, Electrodynamics, Optics and Wave Optics, Modern Physics

Chemistry: Physical Chemistry, Organic Chemistry, Inorganic Chemistry, Mole Concept, Periodic table, Chemical Bonding, Coordination Chemistry

Biology: Reproduction, Genetics and Evolution, Diversity in Living Organisms, Human Physiology, Plant Physiology, Biology and Human Welfare, Biotechnology, Cell Structure and Function 

Make sure you refer the previous years’ papers from the past five years to understand what topics are most recurrent in exams. You should note that topics in Biology will be asked from Botany and Zoology.