If one wishes to visit the temples at their quietest, they should consider a trip during the monsoon season i.e., between June and October. However, one must be prepared for messy trips to the temples in case of heavy afternoon showers. An early start to the day may help you avoid the afternoon rains. As far as the best time in the day to visit is concerned, the Angkor Wat was designed and constructed to face west, thus offering a splendid view of the sunset. Also, to witness of the most beautiful sunrises, one must plan a trip close to an equinox as that is when the sun rises directly over the temple spires.
The best time to visit is between November to March, when the weather is dry and mild.