Hello Aspirant,
See there is no any fixed topic which can be defined as important but yes there are some topics which significantly carries bit higher weightage as compared to other topics. So I am enlisting down these topics from Biology section and then I will also give you the weightage of topics from Physics and Chemistry as well. 

@Biology
Diversity of Living Organisms-14%
Cell: Structure and Function-9%
Human Physiology-20%
Reproduction-9%
Genetics and Evolution-18%
Biology and Human Welfare-9%

@Physics 
Physical-world and measurement-2%
Kinematics-3%
Laws of Motion-3%
Work, Energy and Power-4%
Motion of System of Particles and Rigid Body-5%
Gravitation-2%
Properties of Bulk Matter-3%
Thermodynamics-9%
Behaviour of Perfect Gas and Kinetic Theory-3%
Oscillation & Waves-3%
Electrostatics-9%
Current Electricity-8%
Magnetic Effect of Current & Magnetism-5%
Electromagnetic Induction & Alternating Current-8%
Electromagnetic Waves-5%
Optics-10%
Dual Nature of Matter and Radiation-6%
Atoms & Nuclei-3%
Electronic Devices-9%

@Chemistry
Basic Concepts of Chemistry-1%
Structure of Atom-2%
Classification of Elements & Periodicity in Properties-2%
Chemical Bonding and Molecular Structure-5%
States of Matter: Gases and Liquids-2%
Thermodynamics-8%
Equilibrium-6%
Redox Reactions-3%
s-Block Elements-2%
Some p-Block Elements-2%
Organic Chemistry: Basic Principles & Techniques-4%
Hydrocarbons-3%
Environmental Chemistry-2%
Solid State-2%
Solutions-5%
Electrochemistry-2%
Chemical Kinetics-3%
Surface Chemistry-2%
Isolation of Elements-2%
p-Block Elements-5%
d- and f-Block Elements-4%
Coordination Compounds-9%
Haloalkanes and Haloarenes-3%
Alcohols, Phenols and Ethers-4%
Aldehydes, Ketones and Carboxylic Acids-4%
Organic Compounds containing Nitrogen-2%
Biomolecules-3%
Polymers-3%
Chemistry in Everyday Life-2%


For complete details about important chapters and other weightage related information kindly check out the links given below :-
https://medicine.careers360.com/articles/neet-chapter-wise-weightage-and-important-topics

You can also check out NEET Knockout package for your preparation :-
https://learn.careers360.com/knockout-neet-may/
 
I hope this information helps you.
Good Luck!!