Early to mid-May is about the hottest. The weather is hot and sticky. For those who love the heat, fantastic. Just be aware that the humidity creates the storm clouds, and anytime after lunch there will be a cloudburst. Its like water being poured out of a bucket! And water flows everywhere!

The most popular time to visit is in December when it is cool but it is annoyingly crowded at the temple sights - shoulder to shoulder stuff. The crowd continues till May but tapers off. Although it rains a lot from July till October, it is still good (especially August) to visit as it is less dusty and more greenery, the temple moats are filled, the water in Tonle Sap is clearer - a much prettier sight. And, you don't have to travel so far to get on the boat at the lake since the increased water level means you get to the lake faster.