The CBSE curriculum is considered to be more compact and manageable than that of ICSE till Class 10.
There are five compulsory subjects in CBSE, at the Class 10 level: English, Second Language, Science, Social Studies and Mathematics.
The ICSE curriculum is more comprehensive till Class 10, with significantly higher standards in English. Shakespeare is introduced in Class 9, in many ICSE schools. There is also quite a bit of focus on practical and project work. Apart from that, there is also a sixth subject for which students may opt from a large variety of courses such as Computer Science, Economics, Art, Commercial Applications, Home Science and so on. Students have to appear for separate examinations in History/Geography and Physics/Chemistry/Biology, unlike CBSE where these are clubbed in single papers for Social Studies and Science.
In Class 11 and 12 the ICSE-ISC syllabus is quite similar to that of CBSE. The Science syllabus in particular, is completely aligned with NCERT.
IGCSE and IB are international boards and they have less emphasis on rote learning. IB in particular emphasizes a lot on developing the research abilities of students.
There is a lot of focus on practical and project work. These boards are better suited for candidates aspiring to study abroad after high school. They might not be suitable for those wishing to appear for competitive examinations in India. In the past, IB students have also faced difficulty in the admission process to universities in India. This is one aspect which parents and students might need to look into.
A large number of students from top ISC and CBSE schools have successfully managed to enter top universities abroad, after high school. Some students appear for "AP" tests to help make their application stand out.
The comparison between various boards is often an overblown issue, and the main thing which matters is the school and the approach followed by it towards academics, extra-curricular activities and sports.