Anytime is a good time to visit Kuala Lumpur, but some are better times than others. Ask yourself - are you the type of traveler who likes crowds and the overflowing energy or do you prefer to avoid the peak periods and keep it simple?
With the exception of highland areas, Malaysia has a typical equatorial climate. That means it's hot and humid all year round. Anytime from May to September is considered ideal for a trip to Kuala Lumpur, as there will be less crowds and better weather (i.e., not contant rainfall). Visitors to Kuala Lumpur reach a peak during November to January since that's the winter holiday period as well as time for festivals such as Christmas, New Years' Eve, Hari Raya Puasa (or Ramadan) and Chinese new year. It is during this peak season that hotels will charge their 'high peak' prices.
Temperatures hover between 30 to 32 degrees Celcius during daytime with high levels of humidity. The highest temperature recorded in Kuala Lumpur was 37 degrees Celcius (99 Farenheit), but that does not occur often. Occasional rainshowers are common enough and help balance the heat. Nights are cooler and temperatures drop to 28 or 29 degrees Celcius.
If you intend to head to the islands on the east coast (Tioman, Redang, Perhentian) after or before Kuala Lumpur, keep in mind that the northeast monsoon months are between November to March, and some resorts - even entire islands - may not be open.