Hey,

Its very difficult to say what chapters are important to be very honest. The best way to plan the important chapter is to go through last ten years question bank of CBSE class 12th boards.

It will help you to deal with the important chapters first.

From my experience, you should be focusing more on chapters that involves proofs.

Physics: Magnetism, Light, Modern Physics

Chemistry: Organic should be done in priority, Solid states, surface chemistry, p block, electrochemistry

Biology: All cycles as well as diagrams

Although these are just an assumptions. Try going through as maximum chapters as possible and don't forget to go through last ten year papers.

Hope that helps.

Thank you.