Choosing the right topics for maximum study is very important not only for clearing IITJEE but also for getting a high rank. Lots of topics in IITJEE syllabus require a huge lot of time, incomparable to that required by other topics. Spending too much time on these topics and ignoring other topics leads to drastic failure in rank. Lots of topics in IITJEE are very easy and are of NCERT level. The key to topping IITJEE is to maximize your scoring in these topics which boosts the rank. Many such topics require considerably less time and is much less than the time required by certain other topics with proportionate marks. You should spend more time on these topics and try to earn full marks in such topics. I list a few of these topics:

Modern Physics and Electronics: most of the questions easy and similar to past IITJEE questions.
Wave Optics: most of the questions are similar to past IITJEE questions
Inorganic Chemistry and General Chemistry: most of the questions are of NCERT level and are simple.
Solid State: Most of the questions are very easy.
Probability and Differentiation: most of the questions are simple and NCERT level.
Differential Equations: most of the questions are similar to past IITJEE questions.

By spending legible time on these topics, the scoring in IITJEE will be boosted and only will you clear IITJEE but also get a high rank. But beware, don't neglect there other topics. In topics like mechanics, electricity, physical chemistry and analytical geometry, if the concepts are clearly understood then many of the questions pertaining to these topics can be easily handled. My next topic discusses the marks and the rank which can be expected with those marks.

But don’t try to escape from other topics because of this article because it only tells the most important ones but to get good rank in IITJEE you have to study as much as possible.

All The Best !!!