Anytime.. The temp are never extreme.
Monsoons are good however in case of a flash flood wouldn't be a great experience.
Depends where and what would you like to cover in your trip.
The best festivals like Thrissur pooram is in May, Carnival in cochin is during newyr eve.
Alleppey and house boat best during a monsoon and is quite romantic.
MunnaR again towards November December, waterfalls and dams during the monsoon season.
Beaches all round the year except if there is a cyclone warning.