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.

While the rainy season from mid-May to October can be steamy, the thundershowers tend to cool things off.  Heat builds up, it rains hard and fast and then often quickly clears with a fresh and clean - and cooler - world.  The highest highs are in March, April and May - though the variation is not great.  The coolest mornings are definitely December to February. 

Good historical climate and weather statistics can be found at Siem Reap Weather along with an up-to-the-moment report of weather at the airport.