The best two IISERs for Chemistry are IISER Kolkata and IISER Bhopal. As I'm a student of IISER Kolkata, so I'm not much aware of IISER Bhopal's DCS. But for IISER Kolkata, I can assure you that it is one of the topmost institutes for Chemistry in India. I don't know why, but I've seen people at many different places/website saying that "IISER Kolkata is best known for Physics and Biology Department". But that is quite misleading. Certainly IISER Kolkata has a great Physics and Biology Department, but the Chemistry Department here is its biggest and best department. YES, the best department of IISER Kolkata is not physics department, but CHEMISTRY DEPARTMENT. (You might guess by now that I'm a chemistry person, but I assure you that I'm not.)

Hello!

The best IISER for pure science is pune campus. The ranking goes as follows: pune, mohali, kolkata, then all others. All IISERs are equally good, but the pune campus is the best for every course as per students reviews. The pune campus is one of the oldest IISER. It has many affiliations with foreign universities and the best course structure.

I hope this helps.