Malaysia is tropical and hot year-round at 28–33°C. Rain falls throughout the year, but each coast has a distinct monsoon season. The west coast (Kuala Lumpur, Penang, Langkawi) has its wettest months October–November. The east coast (Perhentian Islands, Tioman) is hit by the northeast monsoon November–February — these islands effectively close.
The best time to visit Kuala Lumpur and Penang is December–February (drier and slightly cooler). Langkawi is best December–April. Borneo (Sabah/Sarawak) is drier March–October. Despite the rain, showers tend to be intense but brief — the rest of the day is often sunny.