Which is most important chapter for Jee main and advanced and what books I have to buy???

Hii

Directly it will be difficult for anyone to tell the important chapter because in these type of prestigious examination almost all chapter play a vital role a well few topics like parabola , hyperbola , ellipse 3D Geometry these are some topics of Mathematics and for other subjects you have to look through the weightage of other topics. You have to buy books like NCERT , SL Arora , HC V , RD Sharma  , Arihant , I.E IRODOV etc are few good books.

Good luck

For jee main all chapters from ncert is important.
Because you will see that from last years papers that cbse ask questions from all chapters.

And for Jee advance
https://www.quora.com/What-are-the-most-important-topics-for-the-JEE-Advanced