Which is the heaviest rocket launched by ISRO till date?

The heaviest rocket launched till date by Indian Space Research Organisation (ISRO) is the Geosynchronous Satellite Launch Vehicle Mark III (GSLV Mk III). You can read about the GSLV Mk III – A Comprehensive Overview of Heavy Lift Launch Vehicle in the given link.

Further readings:

1. Mangalyaan – Mars Mission
2. Reusable Launch Vehicle – Technology Demonstrator (RLV – TD)