please arrangechemical at iit madras ,kgpmech at iit guwahatiece eee at iit guwahati

first go by which branch you like. check out whether you like mech or eee or ece. then it would be easier. depends on whether you want to remain an engineer after 4 years, or want to do upsc or want to go for CAT or simply want to earn money. mech and eee/ece are good choices so you can go for any branch. chemical is full of mugging up and will be very irritating unless you seriously want to pursue it. avoid chemical if you seriously love engi but arent sure of which field. take it if you are trying for upsc or cat, as it will get light by the 3^rd year. ece/eee generally have better access t software and better placements than mech. but if you love mech go for it. look at location of the iit also. consider these choices before going ahead. since i am in ece, i would give the following preference:

ece guw~eee guw>mech guw>>>>>chem madras>~chem kgp

all the best