Most people visit Cambodia during the dry season between November to March. Personally, I think the best time to visit is probably during the wet season between May and October, since the wet season isn’t really that wet. Although it rains heavily nearly every day, the downpours typically only happen late in the afternoon after 4 or 5 pm and therefore do not really impact on your sightseeing or travel plans.  It is also significantly cooler during the rainy season, with the downpours providing some extra cooling. Moreover, during the rainy season, the countryside looks much lusher, moats around the temples and the barrays are full of water, the rice paddies are verdant and the skies contain some impressive cloud formations. Secondly,  the crowds are much thinner during this time.  The above picture, for example, was taken in Angkor Wat at 3pm. At that time, the place was nearly deserted and that with about 2.5 million people visiting the place every year. Finally, because less people are visiting most hotels and hostels are offering discounts and hence travelling becomes significantly cheaper. The only downside is that because of the cloud cover in the early morning and late evening you probably will not be able to get at decent sunset/sunrise picture of Angkor Wat. But that is a small price to pay.