Which are the most important rockets of ISRO?

PSLV and GSLV are the most important rockets of ISRO. Among the two, PSLV is the workhorse of ISRO. You can read about the GSLV Mk III – A Comprehensive Overview of Heavy Lift Launch Vehicle in the given link.

Further readings:

1. Mars Orbiter Mission (MOM) – Payloads, Launch Vehicle, Achievements
2. Reusable Launch Vehicle – Technology Demonstrator (RLV – TD)