During monsoons. You get to the backwaters in their true forms, waterfalls like nowhere and of course the scenic beauty of Kerala which i doubt one can find anywhere else in india.
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.
Winter season between September to March is the best time to visit Kerala. This is the time when Kerala has a comfortable climate and affable rainfall with lower humidity. However, tranquil conduits and racy colonies of Kerala's amazing metropolises, downtime is the perfect time for you, If you wish to explore the alluring lagoons.